Merge branch 'feature/aug2013' into develop

.gitignore

@@ -1,2 +1,4 @@
 *.pyc
 *.*DS_Store
+.*.sw*
+.ropeproject

01-frontpage.md

@@ -0,0 +1,325 @@
+% A Byte of Python
+% Swaroop C H
+% $$date$$
+
+# A Byte of Python #
+
+'A Byte of Python' is a free book on programming using the Python
+language. It serves as a tutorial or guide to the Python language for
+a beginner audience. If all you know about computers is how to save
+text files, then this is the book for you.
+
+This book is written for the latest Python 3, even though Python 2 is
+the commonly found version of Python today (read more about it in
+[Python 2 versus 3 section](#python-2-versus-3)).
+
+## Who Reads A Byte of Python? ##
+
+Here are what people are saying about the book:
+
+> This is the best beginner's tutorial I've ever seen! Thank you for
+> your effort.
+
+> -- _Walt Michalik_ (wmich50-at-theramp-dot-net)
+
+> The best thing i found was "A Byte of Python", which is simply a
+>brilliant book for a beginner. It's well written, the concepts are
+>well explained with self evident examples.
+
+> --
+> [_Syed Talal_](http://littlepancakes.wordpress.com/2010/01/30/python)
+
+> You've made the best Python tutorial I've found on the Net. Great work. Thanks!
+
+> -- _Joshua Robin_ (joshrob-at-poczta-dot-onet-dot-pl)
+
+> Excellent gentle introduction to programming #Python for beginners
+
+> -- [Shan Rajasekaran](https://twitter.com/ShanRajasekaran/status/268910645842423809)
+
+> Hi, I'm from Dominican Republic. My name is Pavel, recently I read
+> your book 'A Byte of Python' and I consider it excellent!!  :). I
+> learnt much from all the examples. Your book is of great help for
+> newbies like me...
+
+> -- _Pavel Simo_ (pavel-dot-simo-at-gmail-dot-com)
+
+> I recently finished reading Byte of Python, and I thought I really
+> ought to thank you. I was very sad to reach the final pages as I now
+> have to go back to dull, tedious oreilly or etc.  manuals for
+> learning about python. Anyway, I really appreciate your book.
+
+> --  _Samuel Young_ (sy-one-three-seven-at-gmail-dot-com)
+
+> Dear Swaroop, I am taking a class from an instructor that has no
+> interest in teaching. We are using Learning Python, second edition,
+> by O'Reilly. It is not a text for beginner without any programming
+> knowledge, and an instructor that should be working in another
+> field.  Thank you very much for your book, without it I would be
+> clueless about Python and programming. Thanks a million, you are
+> able to 'break the message down' to a level that beginners can
+> understand and not everyone can.
+
+> -- _Joseph Duarte_ (jduarte1-at-cfl-dot-rr-dot-com)
+
+> I love your book! It is the greatest Python tutorial ever, and a
+> very useful reference. Brilliant, a true masterpiece! Keep up the
+> good work!
+
+> -- _Chris-André Sommerseth_
+
+> I'm just e-mailing you to thank you for writing Byte of Python
+> online.  I had been attempting Python for a few months prior to
+> stumbling across your book, and although I made limited success with
+> pyGame, I never completed a program.
+
+> Thanks to your simplification of the categories, Python actually
+> seems a reachable goal. It seems like I have finally learned the
+> foundations and I can continue into my real goal, game development.
+
+> ...
+
+> Once again, thanks VERY much for placing such a structured and
+> helpful guide to basic programming on the web.  It shoved me into
+> and out of OOP with an understanding where two text books had
+> failed.
+
+> -- _Matt Gallivan_ (m-underscore-gallivan12-at-hotmail-dot-com)
+
+> I would like to thank you for your book 'A byte of python' which i
+> myself find the best way to learn python. I am a 15 year old i live
+> in egypt my name is Ahmed. Python was my second programming language
+> i learn visual basic 6 at school but didn't enjoy it, however i
+> really enjoyed learning python. I made the addressbook program and i
+> was sucessful. i will try to start make more programs and read
+> python programs (if you could tell me source that would be
+> helpful). I will also start on learning java and if you can tell me
+> where to find a tutorial as good as yours for java that would help
+> me a lot. Thanx.
+
+> -- _Ahmed Mohammed_ (sedo-underscore-91-at-hotmail-dot-com)
+
+> A wonderful resource for beginners wanting to learn more about
+> Python is the 110-page PDF tutorial A Byte of Python by Swaroop C
+> H. It is well-written, easy to follow, and may be the best
+> introduction to Python programming available.
+
+> -- _Drew Ames_ in an article on
+> [Scripting Scribus](http://www.linux.com/feature/126522) published
+> on Linux.com
+
+> Yesterday I got through most of Byte of Python on my Nokia N800 and
+> it's the easiest and most concise introduction to Python I have yet
+> encountered. Highly recommended as a starting point for learning
+> Python.
+
+> -- _Jason Delport_ on his
+> [weblog](http://paxmodept.com/telesto/blogitem.htm?id=627)
+
+> Byte of Vim and Python by @swaroopch is by far the best works in
+> technical writing to me. Excellent reads #FeelGoodFactor
+
+> -- _Surendran_ says in a [tweet](http://twitter.com/suren/status/12840485454)
+
+> "Byte of python" best one by far man
+
+> (in response to the question "Can anyone suggest a good, inexpensive
+> resource for learning the basics of Python? ")
+
+> -- _Justin LoveTrue_ says in a
+> [Facebook community page](http://www.facebook.com/pythonlang/posts/406873916788)
+
+> "The Book Byte of python was very helpful ..Thanks bigtime :)"
+
+> -- [Chinmay](https://twitter.com/a_chinmay/status/258822633741762560)
+
+> Always been a fan of A Byte of Python - made for both new and
+> experienced programmers.
+
+> -- [Patrick Harrington, in a StackOverflow answer](http://stackoverflow.com/a/457785/4869)
+
+> i started learning python few days ago from your book..thanks for
+> such a nice book. it is so well written, you made my life easy..so
+> you found a new fan of yours..thats me :) tons of thanks
+
+> --
+> [Gadadhari](https://twitter.com/Pagal_e_azam/statuses/242865516971163648)
+> [bheem](https://twitter.com/Pagal_e_azam/statuses/242865885256232960)
+
+> Hello Swaroop,
+
+> Thank you ever so much for this book!!
+
+> This book cleared up many questions I had about certain aspects of Python such as object oriented programming.
+> I do not feel like an expert at OO but I know this book helped me on a first step or two.
+
+> I have now written several python programs that actually do real things for me as a system administrator.
+> They are all procedural oriented but they are small by most peoples standards.
+
+> Again, thanks for this book.  Thank you for having it on the web.
+
+> -- Bob
+
+> I wrote you back in 2011 and I was just getting into Python and
+> wanted to thank you for your tutorial "A Byte of Python".  Without
+> it, I would have fallen by the wayside.  Since then I have gone on
+> to program a number of functions in my organization with this
+> language with yet more on the horizon.  I would not call myself an
+> advanced programmer by any stretch but I notice the occasional
+> request for assistance now from others since I started using it.  I
+> discovered, while reading "Byte" why I had ceased studying C and C++
+> and it was because the book given to me started out with an example
+> containing an augmented assignment.  Of course, there was no
+> explanation for this arrangement of operators and I fell on my head
+> trying to make sense of what was on the written page.  As I recall
+> it was a most frustrating exercise which I eventually abandoned.
+> Doesn't mean C or C++ is impossible to learn, or even that I am
+> stupid, but it does mean that the documentation I worked my way
+> through did not define the symbols and words which is an essential
+> part of any instruction. Just as computers will not be able to
+> understand a computer word or computer symbol that is outside the
+> syntax for the language being used, a student new to any field will
+> not grasp his subject if he encounters words or symbols for which
+> there are no definitions.  You get a "blue screen" as it were in
+> either case.  The solution is simple, though: find the word or
+> symbol and get the proper definition or symbol and lo and behold,the
+> computer or student can proceed.  Your book was so well put together
+> that I found very little in it I couldn't grasp.  So, thank you.  I
+> encourage you to continue to include full definitions of terms.  The
+> documentation with Python is good, once you know, (the examples are
+> its strength from what I see) but in many cases it seems that you
+> have to know in order to understand the documentation which to my
+> mind is not what should be.  Third party tutorials express the need
+> for clarification of the documentation and their success largely
+> depends on the words that are used to describe the terminology.  I
+> have recommended your book to many others. Some in Australia, some
+> in the Caribbean and yet others in the US. It fills a niche no
+> others do.  I hope you are doing well and wish you all the success
+> in the future.
+
+> Best Regards,
+
+> -- Nick
+
+> hey, this is ankush(19). I was facing a great difficulty to start
+> with python. I tried a lot of books but all were bulkier and not
+> target oriented; and then i found this lovely one, which made me
+> love python in no time. Thanks a lot for this "beautiful piece of
+> book".
+
+> -- Ankush
+
+> I would like to thank you for your excellent guide on Python. I am a
+> molecular biologist (with little programming background) and for my
+> work I need to handle big datasets of DNA sequences and to analyse
+> microscope images. For both things, programming in python has been
+> useful, if not essential to complete and publish a 6-years project.
+
+> That such a guide is freely available is a clear sign that the
+> forces of evil are not yet ruling the world! :)
+
+> Thank you,
+
+> -- Luca
+
+> Since this is going to be the first language you learn, you should
+> use A Byte of Python. It really gives a proper introduction into
+> programming in Python and it is paced well enough for the average
+> beginner. The most important thing from then on will be actually
+> starting to practice making your own little programs.
+
+> --
+> ["{Unregistered}"](http://www.overclock.net/t/1177951/want-to-learn-programming-where-do-i-start#post_15837176)
+
+> Just to say a loud and happy *thank you very much* for publishing "A
+> Byte of Python" and "A Byte of Vim". Those books were very useful to
+> me four or five years ago when I starting learning
+> programming. Right now I'm developing a project that was a dream for
+> a long, long time and just want to say *thank you*.
+
+> Keep walking. You are a source of motivation.
+
+> All the best.
+
+> -- Jocimar
+
+The book is even used by NASA! It is being used in their
+[Jet Propulsion Laboratory](http://dsnra.jpl.nasa.gov/software/Python/byte-of-python/output/byteofpython_html/)
+with their Deep Space Network project.
+
+## Academic Courses ##
+
+This book is/was being used as instructional material in various
+educational institutions:
+
+- 'Principles of Programming Languages' course at
+  [Vrije Universiteit, Amsterdam](http://www.few.vu.nl/~nsilvis/PPL/2007/index.html)
+- 'Basic Concepts of Computing' course at
+  [University of California, Davis](http://www.cs.ucdavis.edu/courses/exp_course_desc/10.html)
+- 'Programming With Python' course at
+  [Harvard University](http://www.people.fas.harvard.edu/~preshman/python_winter.html)
+- 'Introduction to Programming' course at
+  [University of Leeds](http://www.comp.leeds.ac.uk/acom1900/)
+- 'Introduction to Application Programming' course at
+  [Boston University](http://www.cs.bu.edu/courses/cs108/materials.html)
+- 'Information Technology Skills for Meteorology' course at
+  [University of Oklahoma](http://gentry.metr.ou.edu/byteofpython/)
+- 'Geoprocessing' course at
+  [Michigan State University](http://www.msu.edu/~ashton/classes/825/index.html)
+- 'Multi Agent Semantic Web Systems' course at the
+  [University of Edinburgh](http://homepages.inf.ed.ac.uk/ewan/masws/)
+
+## License ##
+
+This book is licensed under the
+[Creative Commons Attribution-Share Alike 3.0 Unported](http://creativecommons.org/licenses/by-sa/3.0/deed.en_US)
+license.
+
+This means:
+
+- You are free to Share i.e. to copy, distribute and transmit this
+  book
+- You are free to Remix i.e. to adapt this book
+- You are free to use it for commercial purposes
+
+Please note:
+
+- Please do *not* sell electronic or printed copies of the book unless
+  you have clearly and prominently mentioned in the description that
+  these copies are *not* from the original author of this book.
+- Attribution *must* be shown in the introductory description and
+  front page of the document by linking back to
+  <http://swaroopch.com/notes/Python> and clearly indicating that
+  the original text can be fetched from this location.
+- All the code/scripts provided in this book is licensed under the
+  [3-clause BSD License](http://www.opensource.org/licenses/bsd-license.php)
+  unless otherwise noted.
+
+## Read Now ##
+
+You can
+[read the book online](http://swaroopch.com/notes/Python_en-Preface).
+
+## Buy the Book ##
+
+[A printed hardcopy of the book can be purchased](http://swaroopch.com/buybook)
+for your offline reading pleasure, and to support the continued
+development and improvement of this book.
+
+## Download ##
+
+- [PDF](http://files.swaroopch.com/python/byte_of_python.pdf) for
+  desktop viewing
+- [EPUB](http://files.swaroopch.com/python/byte_of_python.epub) for
+  ebook readers
+- [Open source at GitHub](https://github.com/swaroopch/byte_of_python)
+  for cloning the sources
+
+If you wish to support the continued development of this book, please
+consider [buying a hardcopy](http://swaroopch.com/buybook).
+
+## Read the book in your native language ##
+
+If you are interested in reading or contributing translations of this
+book to other human languages, please see the
+[Translations page](#translations).

02-preface.md

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+# Preface #
+
+Python is probably one of the few programming languages which is both
+simple and powerful. This is good for beginners as well as for
+experts, and more importantly, is fun to program with. This book aims
+to help you learn this wonderful language and show how to get things
+done quickly and painlessly - in effect 'The Anti-venom to your
+programming problems'.
+
+## Who This Book Is For ##
+
+This book serves as a guide or tutorial to the Python programming
+language. It is mainly targeted at newbies. It is useful for
+experienced programmers as well.
+
+The aim is that if all you know about computers is how to save text
+files, then you can learn Python from this book. If you have previous
+programming experience, then you can also learn Python from this book.
+
+If you do have previous programming experience, you will be interested
+in the differences between Python and your favorite programming
+language - I have highlighted many such differences. A little warning
+though, Python is soon going to become your favorite programming
+language!
+
+## History Lesson ##
+
+I first started with Python when I needed to write an installer for
+software I had written called 'Diamond' so that I could make the
+installation easy. I had to choose between Python and Perl bindings
+for the Qt library. I did some research on the web and I came across
+[an article by Eric S. Raymond](http://www.python.org/about/success/esr/),
+a famous and respected hacker, where he talked about how Python had
+become his favorite programming language. I also found out that the
+PyQt bindings were more mature compared to Perl-Qt. So, I decided that
+Python was the language for me.
+
+Then, I started searching for a good book on Python. I couldn't find
+any!  I did find some O'Reilly books but they were either too
+expensive or were more like a reference manual than a guide. So, I
+settled for the documentation that came with Python. However, it was
+too brief and small. It did give a good idea about Python but was not
+complete. I managed with it since I had previous programming
+experience, but it was unsuitable for newbies.
+
+About six months after my first brush with Python, I installed the
+(then) latest Red Hat 9.0 Linux and I was playing around with KWord. I
+got excited about it and suddenly got the idea of writing some stuff
+on Python. I started writing a few pages but it quickly became 30
+pages long. Then, I became serious about making it more useful in a
+book form. After a *lot* of rewrites, it has reached a stage where it
+has become a useful guide to learning the Python language.  I consider
+this book to be my contribution and tribute to the open source
+community.
+
+This book started out as my personal notes on Python and I still
+consider it in the same way, although I've taken a lot of effort to
+make it more palatable to others :)
+
+In the true spirit of open source, I have received lots of
+constructive suggestions, criticisms and
+[feedback](#who-reads-a-byte-of-python) from enthusiastic readers
+which has helped me improve this book a lot.
+
+## Status Of The Book ##
+
+- Latest update is in August 2013, converted to much nicer formatted
+  text files using
+  [Jason Blevin's Emacs Markdown Mode](http://jblevins.org/projects/markdown-mode/)
+  and making changes as per numerous readers' suggestions.
+- This book was reformatted in October 2012 using Pandoc to allow
+  generation of ebooks as requested by several users, along with
+  errata fixes and updates.
+- Changes in December 2008 edition (from the earlier major revision in
+  March 2005) was updating for the Python 3.0 release.
+
+The book needs the help of its readers such as yourselves to point out
+any parts of the book which are not good, not comprehensible or are
+simply wrong. Please
+[write to the main author](http://swaroopch.com/contact/) or the
+respective [translators](#translations) with your comments and
+suggestions.
+
+## Official Website ##
+
+The official website of the book is
+<http://swaroopch.com/notes/Python> where you can read the whole book
+online, download the latest versions of the book,
+[buy a printed hard copy](http://swaroopch.com/buybook), and also send
+me feedback.
+
+## Something To Think About ##
+
+> There are two ways of constructing a software design: one way is to
+> make it so simple that there are obviously no deficiencies; the
+> other is to make it so complicated that there are no obvious
+> deficiencies.
+
+> -- C. A. R. Hoare
+
+> Success in life is a matter not so much of talent and opportunity as
+> of concentration and perseverance.
+
+> -- C. W. Wendte

03-intro.md

@@ -0,0 +1,179 @@
+# Introduction #
+
+Python is one of those rare languages which can claim to be both
+*simple* and *powerful*.  You will find yourself pleasantly surprised
+to see how easy it is to concentrate on the solution to the problem
+rather than the syntax and structure of the language you are
+programming in.
+
+The official introduction to Python is:
+
+> Python is an easy to learn, powerful programming language. It has
+> efficient high-level data structures and a simple but effective
+> approach to object-oriented programming. Python's elegant syntax and
+> dynamic typing, together with its interpreted nature, make it an
+> ideal language for scripting and rapid application development in
+> many areas on most platforms.
+
+I will discuss most of these features in more detail in the next
+section.
+
+*Story behind the name:* Guido van Rossum, the creator of the Python
+language, named the language after the BBC show "Monty Python's Flying
+Circus". He doesn't particularly like snakes that kill animals for
+food by winding their long bodies around them and crushing them.
+
+## Features of Python ##
+
+### Simple ###
+
+Python is a simple and minimalistic language. Reading a good Python
+program feels almost like reading English, although very strict
+English! This pseudo-code nature of Python is one of its greatest
+strengths. It allows you to concentrate on the solution to the problem
+rather than the language itself.
+
+### Easy to Learn ###
+
+As you will see, Python is extremely easy to get started with. Python
+has an extraordinarily simple syntax, as already mentioned.
+
+### Free and Open Source ###
+
+Python is an example of a *FLOSS* (Free/Libré and Open Source
+Software). In simple terms, you can freely distribute copies of this
+software, read its source code, make changes to it, and use pieces of
+it in new free programs. FLOSS is based on the concept of a community
+which shares knowledge. This is one of the reasons why Python is so
+good - it has been created and is constantly improved by a community
+who just want to see a better Python.
+
+### High-level Language ###
+
+When you write programs in Python, you never need to bother about the
+low-level details such as managing the memory used by your program,
+etc.
+
+### Portable ###
+
+Due to its open-source nature, Python has been ported to (i.e. changed
+to make it work on) many platforms. All your Python programs can work
+on any of these platforms without requiring any changes at all if you
+are careful enough to avoid any system-dependent features.
+
+You can use Python on Linux, Windows, FreeBSD, Macintosh, Solaris,
+OS/2, Amiga, AROS, AS/400, BeOS, OS/390, z/OS, Palm OS, QNX, VMS,
+Psion, Acorn RISC OS, VxWorks, PlayStation, Sharp Zaurus, Windows CE
+and PocketPC!
+
+You can even use a platform like [Kivy](http://kivy.org) to create
+games for your computer *and* for iPhone, iPad, and Android.
+
+### Interpreted ###
+
+This requires a bit of explanation.
+
+A program written in a compiled language like C or C++ is converted
+from the source language i.e. C or C++ into a language that is spoken
+by your computer (binary code i.e. 0s and 1s) using a compiler with
+various flags and options. When you run the program, the linker/loader
+software copies the program from hard disk to memory and starts
+running it.
+
+Python, on the other hand, does not need compilation to binary. You
+just *run* the program directly from the source code. Internally,
+Python converts the source code into an intermediate form called
+bytecodes and then translates this into the native language of your
+computer and then runs it. All this, actually, makes using Python much
+easier since you don't have to worry about compiling the program,
+making sure that the proper libraries are linked and loaded, etc. This
+also makes your Python programs much more portable, since you can just
+copy your Python program onto another computer and it just works!
+
+### Object Oriented ###
+
+Python supports procedure-oriented programming as well as
+object-oriented programming. In *procedure-oriented* languages, the
+program is built around procedures or functions which are nothing but
+reusable pieces of programs. In *object-oriented* languages, the
+program is built around objects which combine data and
+functionality. Python has a very powerful but simplistic way of doing
+OOP, especially when compared to big languages like C++ or Java.
+
+### Extensible ###
+
+If you need a critical piece of code to run very fast or want to have
+some piece of algorithm not to be open, you can code that part of your
+program in C or C++ and then use it from your Python program.
+
+### Embeddable ###
+
+You can embed Python within your C/C++ programs to give 'scripting'
+capabilities for your program's users.
+
+### Extensive Libraries ###
+
+The Python Standard Library is huge indeed. It can help you do various
+things involving regular expressions,documentation generation, unit
+testing, threading, databases, web browsers, CGI, FTP, email, XML,
+XML-RPC, HTML, WAV files, cryptography, GUI (graphical user
+interfaces), and other system-dependent stuff. Remember, all this is
+always available wherever Python is installed. This is called the
+*Batteries Included* philosophy of Python.
+
+Besides the standard library, there are various other high-quality
+libraries which you can find at the
+[Python Package Index](http://pypi.python.org/pypi).
+
+*Summary:* Python is indeed an exciting and powerful language. It has
+the right combination of performance and features that make writing
+programs in Python both fun and easy.
+
+## Python 2 versus 3 ##
+
+You can ignore this section if you're not interested in the difference
+between Python 2 and Python 3. But please do be aware of which version
+you are using.
+
+This book was rewritten in 2008 for Python 3. It was one of the first
+books to use Python 3. This, unfortunately, resulted in confusion for
+readers who would try to use Python 2 with the Python 3 version of the
+book and vice-versa. But slowly, the world is migrating to Python 3.
+
+So, yes, you will be learning to use Python 3 in this book, even if
+you want to ultimately use Python 2. *Remember that once you have
+properly understood and learn to use either of them, you can easily
+learn the changes between the two versions and adapt easily. The hard
+part is learning programming and understanding the core Python
+language itself. That is our goal in this book, and once you have
+achieved that goal, you can easily use Python 2 or Python 3 depending
+on your situation.*
+
+For details on differences between Python 2 to Python 3, see the
+[Python/3 page on the Ubuntu wiki](https://wiki.ubuntu.com/Python/3).
+
+## What Programmers Say ##
+
+You may find it interesting to read what great hackers like ESR have
+to say about Python:
+
+(@) *Eric S. Raymond* is the author of "The Cathedral and the Bazaar"
+and is also the person who coined the term *Open Source*. He says that
+[Python has become his favorite programming language](http://www.python.org/about/success/esr/). This
+article was the real inspiration for my first brush with Python.
+
+(@) *Bruce Eckel* is the author of the famous *Thinking in Java* and
+*Thinking in C++* books. He says that no language has made him more
+productive than Python. He says that Python is perhaps the only
+language that focuses on making things easier for the programmer. Read
+the [complete interview](http://www.artima.com/intv/aboutme.html) for
+more details.
+
+(@) *Peter Norvig* is a well-known Lisp author and Director of Search
+Quality at Google (thanks to Guido van Rossum for pointing that
+out). He says that
+[writing Python is like writing in pseudocode](https://news.ycombinator.com/item?id=1803815). He
+says that Python has always been an integral part of Google. You can
+actually verify this statement by looking at the
+[Google Jobs](http://www.google.com/jobs/index.html) page which lists
+Python knowledge as a requirement for software engineers.

04-installation.md

@@ -0,0 +1,129 @@
+# Installation #
+
+When we refer to "Python 3" in this book, we will be referring to any
+version of Python equal to or greater than **3.3.2**.
+
+## Installation on Windows ##
+
+Visit <http://www.python.org/download/> and download the latest
+version. The installation is just like any other Windows-based
+software.
+
+*Caution:* When you are given the option of unchecking any "optional"
+ components, don't uncheck any.
+
+### DOS Prompt ###
+
+If you want to be able to use Python from the Windows command line
+i.e. the DOS prompt, then you need to set the PATH variable
+appropriately.
+
+For Windows 2000, XP, 2003 , click on `Control Panel` --- `System` ---
+`Advanced` --- `Environment Variables`. Click on the variable named
+`PATH` in the 'System Variables' section, then select `Edit` and add
+`;C:\Python33` (please verify that this folder exists, it will be
+different for newer versions of Python) to the end of what is already
+there. Of course, use the appropriate directory name.
+
+For older versions of Windows, open the file `C:\AUTOEXEC.BAT` and add
+the line '`PATH=%PATH%;C:\Python33`' (without the quotes) and restart
+the system. For Windows NT, use the `AUTOEXEC.NT` file.
+
+For Windows Vista:
+
+1. Click Start and choose Control Panel
+2. Click System, on the right you'll see "View basic information about
+   your computer"
+3. On the left is a list of tasks, the last of which is "Advanced
+   system settings." Click that.
+4. The Advanced tab of the System Properties dialog box is
+   shown. Click the Environment Variables button on the bottom
+   right.
+5. In the lower box titled "System Variables" scroll down to Path and
+   click the Edit button.
+6. Change your path as need be.
+7. Restart your system. Vista didn't pick up the system path
+   environment variable change until I restarted.
+
+For Windows 7:
+
+1. Right click on Computer from your desktop and select properties or
+   Click Start and choose Control Panel --- System and Security ---
+   System. Click on Advanced system settings on the left and then
+   click on the Advanced tab. At the bottom click on Environment
+   Variables and under System variables, look for the PATH variable,
+   select and then press Edit.
+2. Go to the end of the line under Variable value and append
+   `;C:\Python33`.
+3. If the value was `%SystemRoot%\system32;` It will now become
+   `%SystemRoot%\system32;C:\Python33`
+4. Click OK and you are done. No restart is required.
+
+### Running Python prompt on Windows ###
+
+For Windows users, you can run the interpreter in the command line if
+you have [set the `PATH` variable appropriately](#dos-prompt).
+
+To open the terminal in Windows, click the start button and click
+'Run'. In the dialog box, type `cmd` and press enter key.
+
+Then, type `python` and ensure there are no errors.
+
+## Installation on Mac OS X ##
+
+For Mac OS X users, open the terminal by pressing `Command+Space` keys
+(to open Spotlight search), type `Terminal` and press enter key.
+
+Install [Homebrew](http://mxcl.github.com/homebrew/) by running:
+
+~~~sh
+ruby -e "$(curl -fsSkL raw.github.com/mxcl/homebrew/go)"
+~~~
+
+Then install Python 3 using:
+
+~~~sh
+brew install python3
+~~~
+
+Now, run `python3` and ensure there are no errors.
+
+## Installation on Linux ##
+
+For Linux users, open the terminal by opening the `Terminal`
+application or by pressing `Alt + F2` and entering
+`gnome-terminal`. If that doesn't work, please refer the documentation
+or forums of your particular Linux distribution.
+
+Next, we have to install the `python3` package. For example, on
+Ubuntu, you can use
+[`sudo apt-get install python3`](http://packages.ubuntu.com/search?keywords=python3&searchon=names&suite=all&section=all).
+Please check the documentation or forums of the Linux distribution
+that you have installed for the correct package manager command to
+run.
+
+Once you have finished the installation, run the `python3` and ensure
+there are no errors.
+
+You can see the version of Python on the screen by running:
+
+~~~
+$ python3 -V
+Python 3.3.2
+~~~
+
+*Note:* `$` is the prompt of the shell. It will be different for you
+depending on the settings of the operating system on your computer,
+hence I will indicate the prompt by just the `$` symbol.
+
+*Default in new versions of your distribution?:* Upcoming GNU/Linux
+distributions such as
+[Ubuntu 14.04 LTS are making Python 3 the default version](https://wiki.ubuntu.com/Python/3),
+so check if it is already installed.
+
+## Summary ##
+
+From now on, we will assume that you have Python 3 installed on your
+system.
+
+Next, we will write our first Python 3 program.

04-installation.pd

@@ -1,87 +0,0 @@
-# Installation
-
-## Installation on Windows
-
-Visit <http://www.python.org/download/> and download the latest version. The installation is just like any other Windows-based software.
-
-Caution
-
-:   When you are given the option of unchecking any "optional" components, don't uncheck any.
-
-### DOS Prompt
-
-If you want to be able to use Python from the Windows command line i.e. the DOS prompt, then you need to set the PATH variable appropriately.
-
-For Windows 2000, XP, 2003 , click on `Control Panel` --- `System` --- `Advanced` --- `Environment Variables`. Click on the variable named `PATH` in the 'System Variables' section, then select `Edit` and add `;C:\Python33` (please verify that this folder exists, it will be different for newer versions of Python) to the end of what is already there. Of course, use the appropriate directory name.
-
-For older versions of Windows, open the file `C:\AUTOEXEC.BAT` and add the line '`PATH=%PATH%;C:\Python33`' (without the quotes) and restart the system. For Windows NT, use the `AUTOEXEC.NT` file.
-
-For Windows Vista:
-
-#. Click Start and choose Control Panel
-#. Click System, on the right you’ll see “View basic information about your computer”
-#. On the left is a list of tasks, the last of which is “Advanced system settings.” Click that.
-#. The Advanced tab of the System Properties dialog box is shown. Click the Environment Variables button on the bottom right.
-#. In the lower box titled “System Variables” scroll down to Path and click the Edit button.
-#. Change your path as need be.
-#. Restart your system. Vista didn’t pick up the system path environment variable change until I restarted.
-
-For Windows 7:
-
-#. Right click on Computer from your desktop and select properties or Click Start and choose Control Panel --- System and Security --- System. Click on Advanced system settings on the left and then click on the Advanced tab. At the bottom click on Environment Variables and under System variables, look for the PATH variable, select and then press Edit. 
-#. Go to the end of the line under Variable value and append `;C:\Python33`.
-#. If the value was `%SystemRoot%\system32;` It will now become `%SystemRoot%\system32;C:\Python33`
-#. Click ok and you are done. No restart is required.
-
-### Running Python prompt on Windows
-
-For Windows users, you can run the interpreter in the command line if you have [set the `PATH` variable appropriately](#dos-prompt).
-
-To open the terminal in Windows, click the start button and click 'Run'. In the dialog box, type `cmd` and press enter key.
-
-Then, type `python3 -V` and ensure there are no errors.
-
-## Installation on Mac OS X
-
-For Mac OS X users, open the terminal by pressing `Command+Space` keys (to open Spotlight search), type `Terminal` and press enter key.
-
-Install [Homebrew](http://mxcl.github.com/homebrew/) by running:
-
-~~~
-ruby -e "$(curl -fsSkL raw.github.com/mxcl/homebrew/go)"
-~~~
-
-Then install Python 3 using:
-
-~~~
-brew install python3
-~~~
-
-Now, run `python3 -V` and ensure there are no errors.
-
-## Installation on Linux
-
-For Linux users, open the terminal by opening the `Terminal` application or by pressing `Alt + F2` and entering `gnome-terminal`. If that doesn't work, please refer the documentation or forums of your particular Linux distribution.
-
-Next, we have to install the `python3` package. For example, on Ubuntu, you can use [`sudo apt-get install python3`](http://packages.ubuntu.com/search?keywords=python3&searchon=names&suite=all&section=all). Please check the documentation or forums of the Linux distribution that you have installed for the correct package manager command to run.
-
-Once you have finished the installation, run the `python3 -V` command in a shell and you should see the version of Python on the screen:
-
-~~~
-$ python3 -V
-Python 3.3.0
-~~~
-
-Note
-
-:   `$` is the prompt of the shell. It will be different for you depending on the settings of the operating system on your computer, hence I will indicate the prompt by just the `$` symbol.
-
-Default in new versions of your distribution?
-
-:   Newer distributions such as [Ubuntu 12.10 are making Python 3 the default version](https://wiki.ubuntu.com/Python/3), so check if it is already installed.
-
-## Summary
-
-From now on, we will assume that you have Python 3 installed on your system.
-
-Next, we will write our first Python 3 program.

05-first-steps.md

@@ -0,0 +1,239 @@
+# First Steps #
+
+We will now see how to run a traditional 'Hello World' program in
+Python. This will teach you how to write, save and run Python
+programs.
+
+There are two ways of using Python to run your program - using the
+interactive interpreter prompt or using a source file. We will now see
+how to use both of these methods.
+
+## Using The Interpreter Prompt ##
+
+Open the terminal in your operating system (as discussed previously in
+the [Installation chapter](#installation)) and then open the Python
+prompt by typing `python3` (in GNU/Linux or Mac OS X) or `python` (in
+Windows) and pressing enter key.
+
+Once you have started Python, you should see `>>>` where you can start
+typing stuff. This is called the *Python interpreter prompt*.
+
+At the Python interpreter prompt, type `print('Hello World')` followed
+by the `enter` key. You should see the words `Hello World` as output.
+
+Here is an example of what you should be seeing, when using a Mac OS X
+computer. The details about the Python software will differ based on
+your computer, but the part from the prompt (i.e. from `>>>` onwards)
+should be the same regardless of the operating system.
+
+~~~
+$ python3
+Python 3.3.2 (default, May 30 2013, 19:40:13)
+[GCC 4.2.1 Compatible Apple LLVM 4.2 (clang-425.0.28)] on darwin
+Type "help", "copyright", "credits" or "license" for more information.
+>>> print("hello world")
+hello world
+>>>
+~~~
+
+Notice that Python gives you the output of the line immediately! What
+you just entered is a single Python *statement*. We use `print` to
+(unsurprisingly) print any value that you supply to it. Here, we are
+supplying the text `Hello World` and this is promptly printed to the
+screen.
+
+How to Quit the Interpreter Prompt
+
+:   If you are using a Linux or Unix shell, you can exit the interpreter
+    prompt by pressing `ctrl-d` or entering `exit()` (note: remember to
+    include the parentheses, '()') followed by the `enter` key. If you are
+    using the Windows command prompt, press `ctrl-z` followed by the
+    `enter` key.
+
+## Choosing An Editor ##
+
+We cannot type out our program at the interpreter prompt every time we
+want to run something, so we have to save them in files and can run
+our programs any number of times.
+
+To create our Python source files, we need an editor software where
+you can type and save. A good programmer's editor will make your life
+easier in writing the source files. Hence, the choice of an editor is
+crucial indeed. You have to choose an editor as you would choose a car
+you would buy. A good editor will help you write Python programs
+easily, making your journey more comfortable and helps you reach your
+destination (achieve your goal) in a much faster and safer way.
+
+One of the very basic requirements is *syntax highlighting* where all
+the different parts of your Python program are colorized so that you
+can *see* your program and visualize its running.
+
+If you have no idea where to start, I would recommend using
+[Sublime Text 3](http://www.sublimetext.com/3) software which is
+available on Windows, Mac OS X and Linux. Details in next section.
+
+If you are using Windows, **do not use Notepad** - it is a bad choice
+because it does not do syntax highlighting and also importantly it
+does not support indentation of the text which is very important in
+our case as we will see later. Good editors such as Komodo Edit will
+automatically do this.
+
+If you are an experienced programmer, then you must be already using
+[Vim](http://www.vim.org/) or
+[Emacs](http://www.gnu.org/software/emacs/). Needless to say, these
+are two of the most powerful editors and you will benefit from using
+them to write your Python programs. I personally use both for most of
+my programs, and have even written an
+[entire book on Vim](http://swaroopch.com/notes/vim).
+
+In case you are willing to take the time to learn Vim or Emacs, then I
+highly recommend that you do learn to use either of them as it will be
+very useful for you in the long run. However, as I mentioned before,
+beginners can start with Sublime Text and focus the learning on Python
+rather than the editor at this moment.
+
+To reiterate, please choose a proper editor - it can make writing
+Python programs more fun and easy.
+
+## Sublime Text ##
+
+To use Sublime Text with Python 3, click on `Tools` --- `Build System`
+--- `New Build System` and paste the following into the new file:
+
+~~~
+{
+    "cmd": ["/usr/local/bin/python3", "-u", "$file"],
+    "file_regex": "^[ ]*File \"(...*?)\", line ([0-9]*)",
+    "selector": "source.python",
+    "encoding": "utf8",
+    "path": "/usr/local/Frameworks/Python.framework/Versions/3.3/bin/"
+}
+~~~
+
+and save the file as `Python3`.
+
+Note that you will have to change the `cmd` and `path` in the above
+configuration as per your system's installation.
+
+Next time you open or create a new Python file, be sure to select
+`Tools` --- `Build System` --- `Python3`. After that, you should be
+able to use `Tools` --- `Build` to run your Python file right from
+Sublime Text itself.
+
+## Vim ##
+
+1. Read how to
+   [make Vim a powerful Python IDE by John M Anderson](http://blog.sontek.net/blog/detail/turning-vim-into-a-modern-python-ide).
+2. Try [jedi-vim plugin](https://github.com/davidhalter/jedi-vim)
+3. Also recommended is <https://github.com/swaroopch/dotvim>.
+
+## Emacs ##
+
+1. Install Emacs 24
+2. Run `M-x list-packages`, search for `ELPY` and click on `Install`
+3. Learn how to use ELPY at <https://github.com/jorgenschaefer/elpy/wiki>
+4. Also recommended is <https://github.com/ghoseb/dotemacs>.
+
+## Using A Source File ##
+
+Now let's get back to programming. There is a tradition that whenever
+you learn a new programming language, the first program that you write
+and run is the 'Hello World' program - all it does is just say 'Hello
+World' when you run it. As Simon Cozens (the author of the amazing
+'Beginning Perl' book) puts it, it is the "traditional incantation to
+the programming gods to help you learn the language better."
+
+Start your choice of editor, enter the following program and save it
+as `hello.py`.
+
+If you are using Sublime Text, click on `File` --- `New File`, type
+the lines:
+
+~~~python
+print("Hello World")
+~~~
+
+In Sublime Text, do `File` --- `Save` to save to a file called
+`hello.py`.
+
+Where should you save the file? To any folder for which you know the
+location of the folder. If you don't understand what that means,
+create a new folder and use that location to save and run all your
+Python programs:
+
+- `C:\\py` on Windows
+- `/tmp/py` on GNU/Linux
+- `/tmp/py` on Mac OS X
+
+To create a folder, use the `mkdir` command in the terminal, for
+example, `mkdir /tmp/py`.
+
+Important
+
+:   Always ensure that you give it the file extension of
+    `.py`, for example, `foo.py`.
+
+In Sublime Text, click on `Tools` --- `Build` output printed in a
+small window below.
+
+The best way, though, is to type it in Sublime Text but to use a terminal:
+
+1. Open a terminal as explained in the
+   [Installation chapter](#installation).
+2. *C*hange *d*irectory where you saved the file, for example, `cd
+   /tmp/py`
+3. Run the program by entering the command `python3 hello.py`.
+
+The output is as shown below.
+
+~~~
+$ python3 hello.py
+Hello World
+~~~
+
+If you got the output as shown above, congratulations! - you have
+successfully run your first Python program. You have successfully
+crossed the hardest part of learning programming, which is, getting
+started with your first program!
+
+In case you got an error, please type the above program *exactly* as
+shown above and run the program again. Note that Python is
+case-sensitive i.e. `print` is not the same as `Print` - note the
+lowercase `p` in the former and the uppercase `P` in the latter. Also,
+ensure there are no spaces or tabs before the first character in each
+line - we will [see why this is important later](#indentation).
+
+**How It Works**
+
+A Python program is composed of *statements*. In our first program, we
+have only one statement. In this statement, we call the `print`
+*function* which just prints the text `"Hello World"`. We will learn
+about functions in detail in a [later chapter](#functions) - what you
+should understand now is that whatever you supply in the parentheses
+will be printed back to the screen. In this case, we supply the text
+`"Hello World"`.
+
+## Getting Help ##
+
+If you need quick information about any function or statement in
+Python, then you can use the built-in `help` functionality. This is
+very useful especially when using the interpreter prompt. For example,
+run `help(print)` - this displays the help for the print function
+which is used to print things to the screen.
+
+Note
+
+:   Press `q` to exit the help.
+
+Similarly, you can obtain information about almost anything in
+Python. Use `help()` to learn more about using `help` itself!
+
+In case you need to get help for operators like `return`, then you
+need to put those inside quotes such as `help("return")` so that
+Python doesn't get confused on what we're trying to do.
+
+## Summary ##
+
+You should now be able to write, save and run Python programs at ease.
+
+Now that you are a Python user, let's learn some more Python concepts.

06-basics.md

@@ -0,0 +1,422 @@
+# Basics #
+
+Just printing 'Hello World' is not enough, is it? You want to do more
+than that - you want to take some input, manipulate it and get
+something out of it. We can achieve this in Python using constants and
+variables, and we'll learn some other concepts as well in this
+chapter.
+
+## Comments ##
+
+*Comments* are any text to the right of the `#` symbol and is mainly
+ useful as notes for the reader of the program.
+
+For example:
+
+~~~python
+print('Hello World') # Note that print is a function
+~~~
+
+or:
+
+~~~python
+# Note that print is a function
+print('Hello World')
+~~~
+
+Use as many useful comments as you can in your program to:
+
+- explain assumptions
+- explain important decisions
+- explain important details
+- explain problems you're trying to solve
+- explain problems you're trying to overcome in your program, etc.
+
+[*Code tells you how, comments should tell you why.*](http://www.codinghorror.com/blog/2006/12/code-tells-you-how-comments-tell-you-why.html)
+
+This is useful for readers of your program so that they can easily
+understand what the program is doing. Remember, that person can be
+yourself after six months!
+
+## Literal Constants ##
+
+An example of a literal constant is a number like `5`, `1.23`, or a
+string like `'This is a string'`or `"It's a string!"`. It is called a
+literal because it is *literal* - you use its value literally. The
+number `2` always represents itself and nothing else - it is a
+*constant* because its value cannot be changed. Hence, all these are
+referred to as literal constants.
+
+## Numbers ##
+
+Numbers are mainly of two types - integers and floats.
+
+An examples of an integer is `2` which is just a whole number.
+
+Examples of floating point numbers (or *floats* for short) are `3.23`
+and `52.3E-4`. The `E` notation indicates powers of 10. In this case,
+`52.3E-4` means `52.3 * 10^-4^`.
+
+*Note for Experienced Programmers:* There is no separate `long`
+type. The `int` type can be an integer of any size.
+
+## Strings ##
+
+A string is a *sequence* of *characters*. Strings are basically just a
+bunch of words.
+
+You will be using strings in almost every Python program that you
+write, so pay attention to the following part.
+
+### Single Quote ###
+
+You can specify strings using single quotes such as `'Quote me on
+this'`. All white space i.e. spaces and tabs are preserved as-is.
+
+### Double Quotes ###
+
+Strings in double quotes work exactly the same way as strings in
+single quotes. An example is `"What's your name?"`
+
+### Triple Quotes ###
+
+You can specify multi-line strings using triple quotes - (`"""` or
+`'''`). You can use single quotes and double quotes freely within the
+triple quotes. An example is:
+
+~~~python
+'''This is a multi-line string. This is the first line.
+This is the second line.
+"What's your name?," I asked.
+He said "Bond, James Bond."
+'''
+~~~
+
+### Strings Are Immutable ###
+
+This means that once you have created a string, you cannot change
+it. Although this might seem like a bad thing, it really isn't. We
+will see why this is not a limitation in the various programs that we
+see later on.
+
+*Note for C/C++ Programmers:* There is no separate `char` data type in
+Python. There is no real need for it and I am sure you won't miss it.
+
+*Note for Perl/PHP Programmers:* Remember that single-quoted strings
+and double-quoted strings are the same - they do not differ in any
+way.
+
+### The format method ###
+
+Sometimes we may want to construct strings from other
+information. This is where the `format()` method is useful.
+
+Save the following lines as a file `str_format.py`:
+
+~~~python
+age = 20
+name = 'Swaroop'
+
+print('{0} was {1} years old when he wrote this book'.format(name, age))
+print('Why is {0} playing with that python?'.format(name))
+~~~
+
+Output:
+
+~~~
+$ python3 str_format.py
+Swaroop was 20 years old when he wrote this book
+Why is Swaroop playing with that python?
+~~~
+
+How It Works:
+
+A string can use certain specifications and subsequently, the *format*
+method can be called to substitute those specifications with
+corresponding arguments to the `format` method.
+
+Observe the first usage where we use `{0}` and this corresponds to the
+variable `name` which is the first argument to the format
+method. Similarly, the second specification is `{1}` corresponding to
+`age` which is the second argument to the format method. Note that
+Python starts counting from 0 which means that first position is at
+index 0, second position is at index 1, and so on.
+
+Notice that we could have achieved the same using string
+concatenation: `name + ' is ' + str(age) + ' years old'` but that is
+much uglier and error-prone. Second, the conversion to string would be
+done automatically by the `format` method instead of the explicit
+conversion to strings needed in this case. Third, when using the
+`format` method, we can change the message without having to deal with
+the variables used and vice-versa.
+
+Also note that the numbers are optional, so you could have also
+written as:
+
+~~~python
+age = 20
+name = 'Swaroop'
+
+print('{} was {} years old when he wrote this book'.format(name, age))
+print('Why is {} playing with that python?'.format(name))
+~~~
+
+which will give the same exact output as the previous program.
+
+What Python does in the `format` method is that it substitutes each
+argument value into the place of the specification. There can be more
+detailed specifications such as:
+
+~~~python
+ decimal (.) precision of 3 for float '0.333'
+>>> '{0:.3}'.format(1/3)
+ fill with underscores (_) with the text centered
+ (^) to 11 width '___hello___'
+>>> '{0:_^11}'.format('hello')
+ keyword-based 'Swaroop wrote A Byte of Python'
+>>> '{name} wrote {book}'.format(name='Swaroop', book='A Byte of Python')
+~~~
+
+## Variable ##
+
+Using just literal constants can soon become boring - we need some way
+of storing any information and manipulate them as well. This is where
+*variables* come into the picture. Variables are exactly what the name
+implies - their value can vary, i.e., you can store anything using a
+variable. Variables are just parts of your computer's memory where you
+store some information. Unlike literal constants, you need some method
+of accessing these variables and hence you give them names.
+
+## Identifier Naming ##
+
+Variables are examples of identifiers. *Identifiers* are names given
+to identify *something*. There are some rules you have to follow for
+naming identifiers:
+
+- The first character of the identifier must be a letter of the
+  alphabet (uppercase ASCII or lowercase ASCII or Unicode character)
+  or an underscore ('_').
+- The rest of the identifier name can consist of letters (uppercase
+  ASCII or lowercase ASCII or Unicode character), underscores ('_') or
+  digits (0-9).
+- Identifier names are case-sensitive. For example, `myname` and
+  `myName` are **not** the same. Note the lowercase `n` in the former
+  and the uppercase `N` in the latter.
+- Examples of *valid* identifier names are `i`, `__my_name`,
+  `name_23`. Examples of ''invalid'' identifier names are `2things`,
+  `this is spaced out`, `my-name`, `>a1b2_c3` and
+  `"this_is_in_quotes"`.
+
+## Data Types ##
+
+Variables can hold values of different types called **data
+types**. The basic types are numbers and strings, which we have
+already discussed. In later chapters, we will see how to create our
+own types using [classes](#object-oriented-programming).
+
+## Object ##
+
+Remember, Python refers to anything used in a program as an *object*.
+This is meant in the generic sense. Instead of saying 'the
+*something*', we say 'the *object*'.
+
+*Note for Object Oriented Programming users:* Python is strongly
+object-oriented in the sense that everything is an object including
+numbers, strings and functions.
+
+We will now see how to use variables along with literal
+constants. Save the following example and run the program.
+
+## How to write Python programs ##
+
+Henceforth, the standard procedure to save and run a Python program is
+as follows:
+
+1. Open your editor of choice, such as Sublime Text.
+2. Type the program code given in the example.
+3. Save it as a file with the filename mentioned.
+4. Run the interpreter with the command `python3 program.py` (or
+   `python program.py` on Windows) to run the program.
+
+## Example: Using Variables And Literal Constants ##
+
+~~~python
+ Filename : var.py
+i = 5
+print(i)
+i = i + 1
+print(i)
+
+s = '''This is a multi-line string.
+This is the second line.'''
+print(s)
+~~~
+
+Output:
+
+~~~
+$ python3 var.py
+5
+6
+This is a multi-line string.
+This is the second line.
+~~~
+
+How It Works:
+
+Here's how this program works. First, we assign the literal constant
+value `5` to the variable `i` using the assignment operator
+(`=`). This line is called a statement because it states that
+something should be done and in this case, we connect the variable
+name `i` to the value `5`.  Next, we print the value of `i` using the
+`print` function which, unsurprisingly, just prints the value of the
+variable to the screen.
+
+Then we add `1` to the value stored in `i` and store it back. We then
+print it and expectedly, we get the value `6`.
+
+Similarly, we assign the literal string to the variable `s` and then
+print it.
+
+*Note for static language programmers:* Variables are used by just
+assigning them a value. No declaration or data type definition is
+needed/used.
+
+### Logical And Physical Line ###
+
+A physical line is what you *see* when you write the program. A
+logical line is what *Python sees* as a single statement. Python
+implicitly assumes that each *physical line* corresponds to a *logical
+line*.
+
+An example of a logical line is a statement like `print('Hello
+World')` - if this was on a line by itself (as you see it in an
+editor), then this also corresponds to a physical line.
+
+Implicitly, Python encourages the use of a single statement per line
+which makes code more readable.
+
+If you want to specify more than one logical line on a single physical
+line, then you have to explicitly specify this using a semicolon (`;`)
+which indicates the end of a logical line/statement. For example,
+
+~~~python
+i = 5
+print(i)
+~~~
+
+is effectively same as
+
+~~~python
+i = 5;
+print(i);
+~~~
+
+and the same can be written as
+
+~~~python
+i = 5; print(i);
+~~~
+
+or even
+
+~~~python
+i = 5; print(i)
+~~~
+
+However, I **strongly recommend** that you stick to **writing a
+maximum of a single logical line on each single physical line**. The
+idea is that you should never use the semicolon. In fact, I have
+*never* used or even seen a semicolon in a Python program.
+
+There is one kind of situation where this concept is really useful :
+if you have a long line of code, you can break it into multiple
+physical lines by using the backslash. This is referred to as
+**explicit line joining**:
+
+~~~python
+s = 'This is a string. \
+This continues the string.'
+print(s)
+~~~
+
+This gives the output:
+
+~~~
+This is a string. This continues the string.
+~~~
+
+Similarly,
+
+~~~python
+print\
+(i)
+~~~
+
+is the same as
+
+~~~python
+print(i)
+~~~
+
+Sometimes, there is an implicit assumption where you don't need to use
+a backslash. This is the case where the logical line has a starting
+parentheses, starting square brackets or a starting curly braces but
+not an ending one. This is called **implicit line joining**. You can
+see this in action when we write programs using [lists](#list) in
+later chapters.
+
+### Indentation ###
+
+Whitespace is important in Python. Actually, **whitespace at the
+beginning of the line is important**. This is called
+**indentation**. Leading whitespace (spaces and tabs) at the beginning
+of the logical line is used to determine the indentation level of the
+logical line, which in turn is used to determine the grouping of
+statements.
+
+This means that statements which go together **must** have the same
+indentation. Each such set of statements is called a **block**. We
+will see examples of how blocks are important in later chapters.
+
+One thing you should remember is that wrong indentation can give rise
+to errors. For example:
+
+~~~python
+i = 5
+ print('Value is ', i) # Error! Notice a single space at the start of the line
+print('I repeat, the value is ', i)
+~~~
+
+When you run this, you get the following error:
+
+~~~
+  File "whitespace.py", line 4
+    print('Value is ', i) # Error! Notice a single space at the start of the line
+    ^
+IndentationError: unexpected indent
+~~~
+
+Notice that there is a single space at the beginning of the second
+line. The error indicated by Python tells us that the syntax of the
+program is invalid i.e. the program was not properly written. What
+this means to you is that *you cannot arbitrarily start new blocks of
+statements* (except for the default main block which you have been
+using all along, of course). Cases where you can use new blocks will
+be detailed in later chapters such as the
+[Control Flow](#control-flow).
+
+*How to indent:* Use only spaces for indentation, with a tab stop of 4
+spaces. Good editors like Komodo Edit will automatically do this for
+you. Make sure you use a consistent number of spaces for indentation,
+otherwise your program will show errors.
+
+*Note to static language programmers:* Python will always use
+indentation for blocks and will never use braces. Run `from __future__
+import braces` to learn more.
+
+## Summary ##
+
+Now that we have gone through many nitty-gritty details, we can move
+on to more interesting stuff such as control flow statements. Be sure
+to become comfortable with what you have read in this chapter.

06-basics.pd

@@ -1,314 +0,0 @@
-# Basics
-
-Just printing 'Hello World' is not enough, is it? You want to do more than that - you want to take some input, manipulate it and get something out of it. We can achieve this in Python using constants and variables, and we'll learn some other concepts as well in this chapter.
-
-## Comments
-
-*Comments* are any text to the right of the `#` symbol and is mainly useful as notes for the reader of the program.
-
-For example:
-
-~~~python
-print('Hello World') # Note that print is a function
-~~~
-
-or:
-
-~~~python
-# Note that print is a function
-print('Hello World')
-~~~
-
-Use as many useful comments as you can in your program to:
-
-- explain assumptions
-- explain important decisions
-- explain important details
-- explain problems you're trying to solve
-- explain problems you're trying to overcome in your program, etc.
-
-[*Code tells you how, comments should tell you why.*](http://www.codinghorror.com/blog/2006/12/code-tells-you-how-comments-tell-you-why.html)
-
-This is useful for readers of your program so that they can easily understand what the program is doing. Remember, that person can be yourself after six months!
-
-## Literal Constants
-
-An example of a literal constant is a number like `5`, `1.23`, or a string like `'This is a string'`or `"It's a string!"`. It is called a literal because it is *literal* - you use its value literally. The number `2` always represents itself and nothing else - it is a *constant* because its value cannot be changed. Hence, all these are referred to as literal constants.
-
-## Numbers
-
-Numbers are mainly of two types - integers and floats.
-
-An examples of an integer is `2` which is just a whole number.
-
-Examples of floating point numbers (or *floats* for short) are `3.23` and `52.3E-4`. The `E` notation indicates powers of 10. In this case, `52.3E-4` means `52.3 * 10^-4^`.
-
-Note for Experienced Programmers
-
-:   There is no separate `long` type. The `int` type can be an integer of any size.
-
-## Strings
-
-A string is a *sequence* of *characters*. Strings are basically just a bunch of words.
-
-You will be using strings in almost every Python program that you write, so pay attention to the following part.
-
-### Single Quote
-
-You can specify strings using single quotes such as `'Quote me on this'`. All white space i.e. spaces and tabs are preserved as-is.
-
-### Double Quotes
-
-Strings in double quotes work exactly the same way as strings in single quotes. An example is `"What's your name?"`
-
-### Triple Quotes
-
-You can specify multi-line strings using triple quotes - (`"""` or `'''`). You can use single quotes and double quotes freely within the triple quotes. An example is:
-
-~~~python
-'''This is a multi-line string. This is the first line.
-This is the second line.
-"What's your name?," I asked.
-He said "Bond, James Bond."
-'''
-~~~
-
-### Strings Are Immutable
-
-This means that once you have created a string, you cannot change it. Although this might seem like a bad thing, it really isn't. We will see why this is not a limitation in the various programs that we see later on.
-
-Note for C/C++ Programmers
-
-:   There is no separate `char` data type in Python. There is no real need for it and I am sure you won't miss it.
-
-Note for Perl/PHP Programmers
-
-:   Remember that single-quoted strings and double-quoted strings are the same - they do not differ in any way.
-
-### The format method
-
-Sometimes we may want to construct strings from other information. This is where the `format()` method is useful.
-
-Save the following lines as a file `str_format.py`:
-
-~~~python
-age = 20
-name = 'Swaroop'
-
-print('{0} was {1} years old when he wrote this book'.format(name, age))
-print('Why is {0} playing with that python?'.format(name))
-~~~
-
-Output:
-
-~~~
-$ python3 str_format.py
-Swaroop was 20 years old when he wrote this book
-Why is Swaroop playing with that python?
-~~~
-
-How It Works:
-
-A string can use certain specifications and subsequently, the *format* method can be called to substitute those specifications with corresponding arguments to the `format` method.
-
-Observe the first usage where we use `{0}` and this corresponds to the variable `name` which is the first argument to the format method. Similarly, the second specification is `{1}` corresponding to `age` which is the second argument to the format method. Note that Python starts counting from 0 which means that first position is at index 0, second position is at index 1, and so on.
-
-Notice that we could have achieved the same using string concatenation: `name + ' is ' + str(age) + ' years old'` but that is much uglier and error-prone. Second, the conversion to string would be done automatically by the `format` method instead of the explicit conversion to strings needed in this case. Third, when using the `format` method, we can change the message without having to deal with the variables used and vice-versa.
-
-Also note that the numbers are optional, so you could have also written as:
-
-~~~python
-age = 20
-name = 'Swaroop'
-
-print('{} was {} years old when he wrote this book'.format(name, age))
-print('Why is {} playing with that python?'.format(name))
-~~~
-
-which will give the same exact output as the previous program.
-
-What Python does in the `format` method is that it substitutes each argument value into the place of the specification. There can be more detailed specifications such as:
-
-~~~python
- decimal (.) precision of 3 for float '0.333'
->>> '{0:.3}'.format(1/3)
- fill with underscores (_) with the text centered
- (^) to 11 width '___hello___'
->>> '{0:_^11}'.format('hello')
- keyword-based 'Swaroop wrote A Byte of Python'
->>> '{name} wrote {book}'.format(name='Swaroop', book='A Byte of Python')
-~~~
-
-## Variable
-
-Using just literal constants can soon become boring - we need some way of storing any information and manipulate them as well. This is where *variables* come into the picture. Variables are exactly what the name implies - their value can vary, i.e.,  you can store anything using a variable. Variables are just parts of your computer's memory where you store some information. Unlike literal constants, you need some method of accessing these variables and hence you give them names.
-
-## Identifier Naming
-
-Variables are examples of identifiers. *Identifiers* are names given to identify *something*. There are some rules you have to follow for naming identifiers:
-
-- The first character of the identifier must be a letter of the alphabet (uppercase ASCII or lowercase ASCII or Unicode character) or an underscore ('_').
-- The rest of the identifier name can consist of letters (uppercase ASCII or lowercase ASCII or Unicode character), underscores ('_') or digits (0-9).
-- Identifier names are case-sensitive. For example, `myname` and `myName` are **not** the same. Note the lowercase `n` in the former and the uppercase `N` in the latter.
-- Examples of *valid* identifier names are `i`, `__my_name`, `name_23`. Examples of ''invalid'' identifier names are `2things`, `this is spaced out`, `my-name`, `>a1b2_c3` and `"this_is_in_quotes"`.
-
-## Data Types
-
-Variables can hold values of different types called **data types**. The basic types are numbers and strings, which we have already discussed. In later chapters, we will see how to create our own types using [classes](#object-oriented-programming).
-
-## Object
-
-Remember, Python refers to anything used in a program as an *object*.  This is meant in the generic sense. Instead of saying 'the *something*', we say 'the *object*'.
-
-Note for Object Oriented Programming users
-
-:   Python is strongly object-oriented in the sense that everything is an object including numbers, strings and functions.
-
-We will now see how to use variables along with literal constants. Save the following example and run the program.
-
-## How to write Python programs
-
-Henceforth, the standard procedure to save and run a Python program is as follows:
-
-#. Open your editor of choice, such as Komodo Edit.
-#. Type the program code given in the example.
-#. Save it as a file with the filename mentioned.
-#. Run the interpreter with the command `python3 program.py` to run the program.
-
-## Example: Using Variables And Literal Constants
-
-~~~python
- Filename : var.py
-i = 5
-print(i)
-i = i + 1
-print(i)
-
-s = '''This is a multi-line string.
-This is the second line.'''
-print(s)
-~~~
-
-Output:
-
-~~~
-$ python3 var.py
-5
-6
-This is a multi-line string.
-This is the second line.
-~~~
-
-How It Works:
-
-Here's how this program works. First, we assign the literal constant value `5` to the variable `i` using the assignment operator (`=`). This line is called a statement because it states that something should be done and in this case, we connect the variable name `i` to the value `5`.  Next, we print the value of `i` using the `print` function which, unsurprisingly, just prints the value of the variable to the screen.
-
-Then we add `1` to the value stored in `i` and store it back. We then print it and expectedly, we get the value `6`.
-
-Similarly, we assign the literal string to the variable `s` and then print it.
-
-Note for static language programmers
-
-:   Variables are used by just assigning them a value. No declaration or data type definition is needed/used.
-
-### Logical And Physical Line
-
-A physical line is what you *see* when you write the program. A logical line is what *Python sees* as a single statement. Python implicitly assumes that each *physical line* corresponds to a *logical line*.
-
-An example of a logical line is a statement like `print('Hello World')` - if this was on a line by itself (as you see it in an editor), then this also corresponds to a physical line.
-
-Implicitly, Python encourages the use of a single statement per line which makes code more readable.
-
-If you want to specify more than one logical line on a single physical line, then you have to explicitly specify this using a semicolon (`;`) which indicates the end of a logical line/statement. For example,
-
-~~~python
-i = 5
-print(i)
-~~~
-
-is effectively same as
-
-~~~python
-i = 5;
-print(i);
-~~~
-
-and the same can be written as
-
-~~~python
-i = 5; print(i);
-~~~
-
-or even
-
-~~~python
-i = 5; print(i)
-~~~
-
-However, I **strongly recommend** that you stick to **writing a maximum of a single logical line on each single physical line**. The idea is that you should never use the semicolon. In fact, I have *never* used or even seen a semicolon in a Python program.
-
-There is one kind of situation where this concept is really useful : if you have a  long line of code, you can break it into multiple physical lines by using the backslash. This is referred to as **explicit line joining**:
-
-~~~python
-s = 'This is a string. \
-This continues the string.'
-print(s)
-~~~
-
-This gives the output:
-
-~~~
-This is a string. This continues the string.
-~~~
-
-Similarly,
-
-~~~python
-print\
-(i)
-~~~
-
-is the same as
-
-~~~python
-print(i)
-~~~
-
-Sometimes, there is an implicit assumption where you don't need to use a backslash. This is the case where the logical line has a starting parentheses, starting square brackets or a starting curly braces but not an ending one. This is called **implicit line joining**. You can see this in action when we write programs using [lists](#list) in later chapters.
-
-### Indentation
-
-Whitespace is important in Python. Actually, **whitespace at the beginning of the line is important**. This is called **indentation**. Leading whitespace (spaces and tabs) at the beginning of the logical line is used to determine the indentation level of the logical line, which in turn is used to determine the grouping of statements.
-
-This means that statements which go together **must** have the same indentation. Each such set of statements is called a **block**. We will see examples of how blocks are important in later chapters.
-
-One thing you should remember is that wrong indentation can give rise to errors. For example:
-
-~~~python
-i = 5
- print('Value is ', i) # Error! Notice a single space at the start of the line
-print('I repeat, the value is ', i)
-~~~
-
-When you run this, you get the following error:
-
-~~~
-  File "whitespace.py", line 4
-    print('Value is ', i) # Error! Notice a single space at the start of the line
-    ^
-IndentationError: unexpected indent
-~~~
-
-Notice that there is a single space at the beginning of the second line. The error indicated by Python tells us that the syntax of the program is invalid i.e. the program was not properly written. What this means to you is that *you cannot arbitrarily start new blocks of statements* (except for the default main block which you have been using all along, of course). Cases where you can use new blocks will be detailed in later chapters such as the [Control Flow](#control-flow).
-
-How to indent
-
-:   Use only spaces for indentation, with a tab stop of 4 spaces. Good editors like Komodo Edit will automatically do this for you. Make sure you use a consistent number of spaces for indentation, otherwise your program will show errors.
-
-Note to static language programmers
-
-:   Python will always use indentation for blocks and will never use braces. Run `from __future__ import braces` to learn more.
-
-## Summary
-
-Now that we have gone through many nitty-gritty details, we can move on to more interesting stuff such as control flow statements. Be sure to become comfortable with what you have read in this chapter.

07-operators-expressions.md

@@ -0,0 +1,367 @@
+# Operators and Expressions #
+
+Most statements (logical lines) that you write will contain
+*expressions*. A simple example of an expression is `2 + 3`. An
+expression can be broken down into operators and operands.
+
+*Operators* are functionality that do something and can be represented
+by symbols such as `+` or by special keywords. Operators require some
+data to operate on and such data is called *operands*. In this case,
+`2` and `3` are the operands.
+
+## Operators ##
+
+We will briefly take a look at the operators and their usage:
+
+Note that you can evaluate the expressions given in the examples using
+the interpreter interactively. For example, to test the expression
+`2 + 3`, use the interactive Python interpreter prompt:
+
+~~~python
+>>> 2 + 3
+5
+>>> 3 * 5
+15
+>>>
+~~~
+
+`+` (plus)
+
+:   Adds two objects
+
+    `3 + 5` gives `8`. `'a' + 'b'` gives `'ab'`.
+
+`-` (minus)
+
+:   Gives the subtraction of one number from the other; if the first
+    operand is absent it is assumed to be zero.
+
+    `-5.2` gives a negative number and `50 - 24` gives `26`.
+
+`*` (multiply)
+
+:   Gives the multiplication of the two numbers or returns the string
+    repeated that many times.
+
+    `2 * 3` gives `6`. `'la' * 3` gives `'lalala'`.
+
+`**` (power)
+
+:   Returns x to the power of y
+
+    `3 ** 4` gives `81` (i.e. `3 * 3 * 3 * 3`)
+
+`/` (divide)
+
+:   Divide x by y
+
+    `4 / 3` gives `1.3333333333333333`.
+
+`//` (floor division)
+
+:   Returns the floor of the quotient
+
+    `4 // 3` gives `1`.
+
+`%` (modulo)
+
+:   Returns the remainder of the division
+
+    `8 % 3` gives `2`. `-25.5 % 2.25` gives `1.5`.
+
+`<<` (left shift)
+
+:   Shifts the bits of the number to the left by the number of bits
+    specified. (Each number is represented in memory by bits or binary
+    digits i.e. 0 and 1)
+
+    `2 << 2` gives `8`. `2` is represented by `10` in bits.
+
+    Left shifting by 2 bits gives `1000` which represents the decimal
+    `8`.
+
+`>>` (right shift)
+
+:   Shifts the bits of the number to the right by the number of bits
+    specified.
+
+    `11 >> 1` gives `5`.
+
+    `11` is represented in bits by `1011` which when right shifted by 1 bit
+    gives `101`which is the decimal `5`.
+
+`&` (bit-wise AND)
+
+:   Bit-wise AND of the numbers
+
+    `5 & 3` gives `1`.
+
+`|` (bit-wise OR)
+
+:   Bitwise OR of the numbers
+
+    `5 | 3` gives `7`
+
+`^` (bit-wise XOR)
+
+:   Bitwise XOR of the numbers
+
+    `5 ^ 3` gives `6`
+
+`~` (bit-wise invert)
+
+:   The bit-wise inversion of x is -(x+1)
+
+    `~5` gives `-6`.
+
+`<` (less than)
+
+:   Returns whether x is less than y. All comparison operators return
+    `True` or `False`. Note the capitalization of these names.
+
+    `5 &lt; 3` gives `False` and `3 &lt; 5` gives `True`.
+
+    Comparisons can be chained arbitrarily: `3 &lt; 5 &lt; 7` gives
+    `True`.
+
+`>` (greater than)
+
+:   Returns whether x is greater than y
+
+    `5 &gt; 3` returns `True`. If both operands are numbers, they are
+    first converted to a common type. Otherwise, it always returns
+    `False`.
+
+`<=` (less than or equal to)
+
+:   Returns whether x is less than or equal to y
+
+    `x = 3; y = 6; x &lt;= y` returns `True`.
+
+`>=` (greater than or equal to)
+
+:   Returns whether x is greater than or equal to y
+
+    `x = 4; y = 3; x &gt;= 3` returns `True`.
+
+`==` (equal to)
+
+:   Compares if the objects are equal
+
+    `x = 2; y = 2; x == y` returns `True`.
+
+    `x = 'str'; y = 'stR'; x == y` returns `False`.
+
+    `x = 'str'; y = 'str'; x == y` returns `True`.
+
+`!=` (not equal to)
+
+:   Compares if the objects are not equal
+
+    `x = 2; y = 3; x != y` returns `True`.
+
+`not` (boolean NOT)
+
+:   If x is `True`, it returns `False`. If x is `False`, it returns
+    `True`.
+
+    `x = True; not x` returns `False`.
+
+`and` (boolean AND)
+
+:   `x and y` returns `False` if x is `False`, else it returns
+    evaluation of y
+
+    `x = False; y = True; x and y` returns `False` since x is False.
+    In this case, Python will not evaluate y since it knows that the
+    left hand side of the 'and' expression is `False` which implies
+    that the whole expression will be `False` irrespective of the
+    other values. This is called short-circuit evaluation.
+
+`or` (boolean OR)
+
+:   If x is `True`, it returns True, else it returns evaluation of y
+
+    `x = True; y = False; x or y` returns `True`. Short-circuit
+    evaluation applies here as well.
+
+### Shortcut for math operation and assignment ###
+
+It is common to run a math operation on a variable and then assign the
+result of the operation back to the variable, hence there is a
+shortcut for such expressions:
+
+You can write:
+
+~~~python
+a = 2
+a = a * 3
+~~~
+
+
+as:
+
+~~~python
+a = 2
+a *= 3
+~~~
+
+Notice that `var = var operation expression` becomes `var operation= expression`.
+
+## Evaluation Order ##
+
+If you had an expression such as `2 + 3 * 4`, is the addition done
+first or the multiplication? Our high school maths tells us that the
+multiplication should be done first. This means that the
+multiplication operator has higher precedence than the addition
+operator.
+
+The following table gives the precedence table for Python, from the
+lowest precedence (least binding) to the highest precedence (most
+binding). This means that in a given expression, Python will first
+evaluate the operators and expressions lower in the table before the
+ones listed higher in the table.
+
+The following table, taken from the
+[Python reference manual](http://docs.python.org/3/reference/expressions.html#operator-precedence),
+is provided for the sake of completeness. It is far better to use
+parentheses to group operators and operands appropriately in order to
+explicitly specify the precedence. This makes the program more
+readable. See
+[Changing the Order of Evaluation](#changing-the-order-of-evaluation)
+below for details.
+
+`lambda`
+
+:   Lambda Expression
+
+`if - else`
+
+:   Conditional expression
+
+`or`
+
+:   Boolean OR
+
+`and`
+
+:   Boolean AND
+
+`not x`
+
+:   Boolean NOT
+
+`in, not in, is, is not, <, <=, >, >=, !=, ==`
+
+:   Comparisons, including membership tests and identity tests
+
+`|`
+
+:   Bitwise OR
+
+`^`
+
+:   Bitwise XOR
+
+`&`
+
+:   Bitwise AND
+
+`<<, >>`
+
+:   Shifts
+
+`+, -`
+
+:   Addition and subtraction
+
+`*, /, //, %`
+
+:   Multiplication, Division, Floor Division and Remainder
+
+`+x, -x, ~x`
+
+:   Positive, Negative, bitwise NOT
+
+`**`
+
+:   Exponentiation
+
+`x[index], x[index:index], x(arguments...), x.attribute`
+
+:   Subscription, slicing, call, attribute reference
+
+`(expressions...), [expressions...], {key: value...}, {expressions...}`
+
+:   Binding or tuple display, list display, dictionary display, set display
+
+The operators which we have not already come across will be explained
+in later chapters.
+
+Operators with the *same precedence* are listed in the same row in the
+above table. For example, `+` and `-` have the same precedence.
+
+## Changing the Order Of Evaluation ##
+
+To make the expressions more readable, we can use parentheses. For
+example, `2 + (3 * 4)` is definitely easier to understand than `2 +
+3 * 4` which requires knowledge of the operator precedences. As with
+everything else, the parentheses should be used reasonably (do not
+overdo it) and should not be redundant, as in `(2 + (3 * 4))`.
+
+There is an additional advantage to using parentheses - it helps us to
+change the order of evaluation. For example, if you want addition to
+be evaluated before multiplication in an expression, then you can
+write something like `(2 + 3) * 4`.
+
+## Associativity ##
+
+Operators are usually associated from left to right. This means that
+operators with the same precedence are evaluated in a left to right
+manner. For example, `2 + 3 + 4` is evaluated as `(2 + 3) + 4`. Some
+operators like assignment operators have right to left associativity
+i.e. `a = b = c` is treated as `a = (b = c)`.
+
+## Expressions ##
+
+Example (save as `expression.py`):
+
+~~~python
+length = 5
+breadth = 2
+
+area = length * breadth
+print('Area is', area)
+print('Perimeter is', 2 * (length + breadth))
+~~~
+
+Output:
+
+~~~
+$ python3 expression.py
+Area is 10
+Perimeter is 14
+~~~
+
+How It Works:
+
+The length and breadth of the rectangle are stored in variables by the
+same name. We use these to calculate the area and perimeter of the
+rectangle with the help of expressions. We store the result of the
+expression `length * breadth` in the variable `area` and then print it
+using the `print` function. In the second case, we directly use the
+value of the expression `2 * (length + breadth)` in the print
+function.
+
+Also, notice how Python 'pretty-prints' the output. Even though we
+have not specified a space between `'Area is'` and the variable
+`area`, Python puts it for us so that we get a clean nice output and
+the program is much more readable this way (since we don't need to
+worry about spacing in the strings we use for output). This is an
+example of how Python makes life easy for the programmer.
+
+## Summary ##
+
+We have seen how to use operators, operands and expressions - these
+are the basic building blocks of any program. Next, we will see how to
+make use of these in our programs using statements.

07-operators-expressions.pd

@@ -1,328 +0,0 @@
-# Operators and Expressions
-
-Most statements (logical lines) that you write will contain *expressions*. A simple example of an expression is `2 + 3`. An expression can be broken down into operators and operands.
-
-*Operators* are functionality that do something and can be represented by symbols such as `+` or by special keywords. Operators require some data to operate on and such data is called *operands*. In this case, `2` and `3` are the operands.
-
-## Operators
-
-We will briefly take a look at the operators and their usage:
-
-Note that you can evaluate the expressions given in the examples using the interpreter interactively. For example, to test the expression `2 + 3`, use the interactive Python interpreter prompt:
-
-~~~python
->>> 2 + 3
-5
->>> 3 * 5
-15
->>>
-~~~
-
-`+` (plus)
-
-:   Adds two objects
-
-    `3 + 5` gives `8`. `'a' + 'b'` gives `'ab'`.
-
-`-` (minus)
-
-:   Gives the subtraction of one number from the other; if the first operand is absent it is assumed to be zero.
-
-    `-5.2` gives a negative number and `50 - 24` gives `26`.
-
-`*` (multiply)
-
-:   Gives the multiplication of the two numbers or returns the string repeated that many times.
-
-    `2 * 3` gives `6`. `'la' * 3` gives `'lalala'`.
-
-`**` (power)
-
-:   Returns x to the power of y
-
-    `3 ** 4` gives `81` (i.e. `3 * 3 * 3 * 3`)
-
-`/` (divide)
-
-:   Divide x by y
-
-    `4 / 3` gives `1.3333333333333333`.
-
-`//` (floor division)
-
-:   Returns the floor of the quotient
-
-    `4 // 3` gives `1`.
-
-`%` (modulo)
-
-:   Returns the remainder of the division
-
-    `8 % 3` gives `2`. `-25.5 % 2.25` gives `1.5`.
-
-`<<` (left shift)
-
-:   Shifts the bits of the number to the left by the number of bits specified. (Each number is represented in memory by bits or binary digits i.e. 0 and 1)
-
-    `2 << 2` gives `8`. `2` is represented by `10` in bits.
-
-    Left shifting by 2 bits gives `1000` which represents the decimal `8`.
-
-`>>` (right shift)
-
-:   Shifts the bits of the number to the right by the number of bits specified.
-
-    `11 >> 1` gives `5`.
-
-    `11` is represented in bits by `1011` which when right shifted by 1 bit
-    gives `101`which is the decimal `5`.
-
-`&` (bit-wise AND)
-
-:   Bit-wise AND of the numbers
-
-    `5 & 3` gives `1`.
-
-`|` (bit-wise OR)
-
-:   Bitwise OR of the numbers
-
-    `5 | 3` gives `7`
-
-`^` (bit-wise XOR)
-
-:   Bitwise XOR of the numbers
-
-    `5 ^ 3` gives `6`
-
-`~` (bit-wise invert)
-
-:   The bit-wise inversion of x is -(x+1)
-
-    `~5` gives `-6`.
-
-`<` (less than)
-
-:   Returns whether x is less than y. All comparison operators return `True` or `False`. Note the capitalization of these names.
-
-    `5 &lt; 3` gives `False` and `3 &lt; 5` gives `True`.
-
-    Comparisons can be chained arbitrarily: `3 &lt; 5 &lt; 7` gives `True`.
-
-`>` (greater than)
-
-:   Returns whether x is greater than y
-
-    `5 &gt; 3` returns `True`. If both operands are numbers, they are first converted to a common type. Otherwise, it always returns `False`.
-
-`<=` (less than or equal to)
-
-:   Returns whether x is less than or equal to y
-
-    `x = 3; y = 6; x &lt;= y` returns `True`.
-
-`>=` (greater than or equal to)
-
-:   Returns whether x is greater than or equal to y
-
-    `x = 4; y = 3; x &gt;= 3` returns `True`.
-
-`==` (equal to)
-
-:   Compares if the objects are equal
-
-    `x = 2; y = 2; x == y` returns `True`.
-
-    `x = 'str'; y = 'stR'; x == y` returns `False`.
-
-    `x = 'str'; y = 'str'; x == y` returns `True`.
-
-`!=` (not equal to)
-
-:   Compares if the objects are not equal
-
-    `x = 2; y = 3; x != y` returns `True`.
-
-`not` (boolean NOT)
-
-:   If x is `True`, it returns `False`. If x is `False`, it returns `True`.
-
-    `x = True; not x` returns `False`.
-
-`and` (boolean AND)
-
-:   `x and y` returns `False` if x is `False`, else it returns evaluation of y
-
-    `x = False; y = True; x and y` returns `False` since x is False. In this case, Python will not evaluate y since it knows that the left hand side of the 'and' expression is `False` which implies that the whole expression will be `False` irrespective of the other values. This is called short-circuit evaluation.
-
-`or` (boolean OR)
-
-:   If x is `True`, it returns True, else it returns evaluation of y
-
-    `x = True; y = False; x or y` returns `True`. Short-circuit evaluation applies here as well.
-
-### Shortcut for math operation and assignment
-
-It is common to run a math operation on a variable and then assign the result of the operation back to the variable, hence there is a shortcut for such expressions:
-
-You can write:
-
-~~~python
-a = 2
-a = a * 3
-~~~
-
-
-as:
-
-~~~python
-a = 2
-a *= 3
-~~~
-
-Notice that `var = var operation expression` becomes `var operation= expression`.
-
-## Evaluation Order
-
-If you had an expression such as `2 + 3 * 4`, is the addition done first or the multiplication? Our high school maths tells us that the multiplication should be done first. This means that the multiplication operator has higher precedence than the addition operator.
-
-The following table gives the precedence table for Python, from the lowest precedence (least binding) to the highest precedence (most binding). This means that in a given expression, Python will first evaluate the operators and expressions lower in the table before the ones listed higher in the table.
-
-The following table, taken from the [Python reference manual](http://docs.python.org/py3k/reference/expressions.html#summary), is provided for the sake of completeness. It is far better to use parentheses to group operators and operands appropriately in order to explicitly specify the precedence. This makes the program more readable. See [Changing the Order of Evaluation](#changing-the-order-of-evaluation) below for details.
-
-`lambda`
-
-:   Lambda Expression
-
-`or`
-
-:   Boolean OR
-
-`and`
-
-:   Boolean AND
-
-`not x`
-
-:   Boolean NOT
-
-`in, not in`
-
-:   Membership tests
-
-`is, is not`
-
-:   Identity tests
-
-`<, <=, >, >=, !=, ==`
-
-:   Comparisons
-
-`|`
-
-:   Bitwise OR
-
-`^`
-
-:   Bitwise XOR
-
-`&`
-
-:   Bitwise AND
-
-`<<, >>`
-
-:   Shifts
-
-`+, -`
-
-:   Addition and subtraction
-
-`*, /, //, %`
-
-:   Multiplication, Division, Floor Division and Remainder
-
-`+x, -x`
-
-:   Positive, Negative
-
-`~x`
-
-:   Bitwise NOT
-
-`**`
-
-:   Exponentiation
-
-`x.attribute`
-
-:   Attribute reference
-
-`x[index]`
-
-:   Subscription
-
-`x[index1:index2]`
-
-:   Slicing
-
-`f(arguments ...)`
-
-:   Function call
-
-`(expressions, ...)`
-
-:   Binding or tuple display
-
-`[expressions, ...]`
-
-:   List display
-
-`{key:datum, ...}`
-
-:   Dictionary display
-
-The operators which we have not already come across will be explained in later chapters.
-
-Operators with the *same precedence* are listed in the same row in the above table. For example, `+` and `-` have the same precedence.
-
-## Changing the Order Of Evaluation
-
-To make the expressions more readable, we can use parentheses. For example, `2 + (3 * 4)` is definitely easier to understand than `2 + 3 * 4` which requires knowledge of the operator precedences. As with everything else, the parentheses should be used reasonably (do not overdo it) and should not be redundant, as in `(2 + (3 * 4))`.
-
-There is an additional advantage to using parentheses - it helps us to change the order of evaluation. For example, if you want addition to be evaluated before multiplication in an expression, then you can write something like `(2 + 3) * 4`.
-
-## Associativity
-
-Operators are usually associated from left to right. This means that operators with the same precedence are evaluated in a left to right manner. For example, `2 + 3 + 4` is evaluated as `(2 + 3) + 4`. Some operators like assignment operators have right to left associativity i.e. `a = b = c` is treated as `a = (b = c)`.
-
-## Expressions
-
-Example (save as `expression.py`):
-
-~~~python
-length = 5
-breadth = 2
-
-area = length * breadth
-print('Area is', area)
-print('Perimeter is', 2 * (length + breadth))
-~~~
-
-Output:
-
-~~~
-$ python3 expression.py
-Area is 10
-Perimeter is 14
-~~~
-
-How It Works:
-
-The length and breadth of the rectangle are stored in variables by the same name. We use these to calculate the area and perimeter of the rectangle with the help of expressions. We store the result of the expression `length * breadth` in the variable `area` and then print it using the `print` function. In the second case, we directly use the value of the expression `2 * (length + breadth)` in the print function.
-
-Also, notice how Python 'pretty-prints' the output. Even though we have not specified a space between `'Area is'` and the variable `area`, Python puts it for us so that we get a clean nice output and the program is much more readable this way (since we don't need to worry about spacing in the strings we use for output). This is an example of how Python makes life easy for the programmer.
-
-## Summary
-
-We have seen how to use operators, operands and expressions - these are the basic building blocks of any program. Next, we will see how to make use of these in our programs using statements.

08-control-flow.md

@@ -0,0 +1,391 @@
+# Control Flow #
+
+In the programs we have seen till now, there has always been a series
+of statements faithfully executed by Python in exact top-down
+order. What if you wanted to change the flow of how it works? For
+example, you want the program to take some decisions and do different
+things depending on different situations, such as printing 'Good
+Morning' or 'Good Evening' depending on the time of the day?
+
+As you might have guessed, this is achieved using control flow
+statements. There are three control flow statements in Python - `if`,
+`for` and `while`.
+
+## The if statement ##
+
+The `if` statement is used to check a condition: *if* the condition is
+true, we run a block of statements (called the *if-block*), *else* we
+process another block of statements (called the *else-block*). The
+*else* clause is optional.
+
+Example (save as `if.py`):
+
+~~~python
+number = 23
+guess = int(input('Enter an integer : '))
+
+if guess == number:
+    print('Congratulations, you guessed it.') # New block starts here
+    print('(but you do not win any prizes!)') # New block ends here
+elif guess < number:
+    print('No, it is a little higher than that') # Another block
+    # You can do whatever you want in a block ...
+else:
+    print('No, it is a little lower than that')
+    # you must have guessed > number to reach here
+
+print('Done')
+# This last statement is always executed, after the if statement is executed
+~~~
+
+Output:
+
+~~~
+$ python3 if.py
+Enter an integer : 50
+No, it is a little lower than that
+Done
+
+$ python3 if.py
+Enter an integer : 22
+No, it is a little higher than that
+Done
+
+$ python3 if.py
+Enter an integer : 23
+Congratulations, you guessed it.
+(but you do not win any prizes!)
+Done
+~~~
+
+How It Works:
+
+In this program, we take guesses from the user and check if it is the
+number that we have. We set the variable `number` to any integer we
+want, say `23`. Then, we take the user's guess using the `input()`
+function. Functions are just reusable pieces of programs. We'll read
+more about them in the [next chapter](#functions).
+
+We supply a string to the built-in `input` function which prints it to
+the screen and waits for input from the user. Once we enter something
+and press `enter` key, the `input()` function returns what we entered,
+as a string. We then convert this string to an integer using `int` and
+then store it in the variable `guess`. Actually, the `int` is a class
+but all you need to know right now is that you can use it to convert a
+string to an integer (assuming the string contains a valid integer in
+the text).
+
+Next, we compare the guess of the user with the number we have
+chosen. If they are equal, we print a success message. Notice that we
+use indentation levels to tell Python which statements belong to which
+block. This is why indentation is so important in Python. I hope you
+are sticking to the "consistent indentation" rule. Are you?
+
+Notice how the `if` statement contains a colon at the end - we are
+indicating to Python that a block of statements follows.
+
+Then, we check if the guess is less than the number, and if so, we
+inform the user that they must guess a little higher than that. What
+we have used here is the `elif` clause which actually combines two
+related `if else-if else` statements into one combined `if-elif-else`
+statement. This makes the program easier and reduces the amount of
+indentation required.
+
+The `elif` and `else` statements must also have a colon at the end of
+the logical line followed by their corresponding block of statements
+(with proper indentation, of course)
+
+You can have another `if` statement inside the if-block of an `if`
+statement and so on - this is called a nested `if` statement.
+
+Remember that the `elif` and `else` parts are optional. A minimal
+valid `if` statement is:
+
+~~~python
+if True:
+    print('Yes, it is true')
+~~~
+
+After Python has finished executing the complete `if` statement along
+with the associated `elif` and `else` clauses, it moves on to the next
+statement in the block containing the `if` statement. In this case, it
+is the main block (where execution of the program starts), and the
+next statement is the `print('Done')` statement. After this, Python
+sees the ends of the program and simply finishes up.
+
+Even though this is a very simple program, I have been pointing out a
+lot of things that you should notice. All these are pretty
+straightforward (and surprisingly simple for those of you from C/C++
+backgrounds). You will need to become aware of all these things
+initially, but after some practice you will become comfortable with
+them, and it will all feel 'natural' to you.
+
+Note for C/C++ Programmers
+
+:   There is no `switch` statement in Python. You can use an
+    `if..elif..else` statement to do the same thing (and in some cases,
+    use a [dictionary](#dictionary) to do it quickly)
+
+## The while Statement ##
+
+The `while` statement allows you to repeatedly execute a block of
+statements as long as a condition is true. A `while` statement is an
+example of what is called a *looping* statement. A `while` statement
+can have an optional `else` clause.
+
+Example (save as `while.py`):
+
+~~~python
+number = 23
+running = True
+
+while running:
+    guess = int(input('Enter an integer : '))
+
+    if guess == number:
+        print('Congratulations, you guessed it.')
+        running = False # this causes the while loop to stop
+    elif guess < number:
+        print('No, it is a little higher than that.')
+    else:
+        print('No, it is a little lower than that.')
+else:
+    print('The while loop is over.')
+    # Do anything else you want to do here
+
+print('Done')
+~~~
+
+Output:
+
+~~~
+$ python3 while.py
+Enter an integer : 50
+No, it is a little lower than that.
+Enter an integer : 22
+No, it is a little higher than that.
+Enter an integer : 23
+Congratulations, you guessed it.
+The while loop is over.
+Done
+~~~
+
+How It Works:
+
+In this program, we are still playing the guessing game, but the
+advantage is that the user is allowed to keep guessing until he
+guesses correctly - there is no need to repeatedly run the program for
+each guess, as we have done in the previous section. This aptly
+demonstrates the use of the `while` statement.
+
+We move the `input` and `if` statements to inside the `while` loop and
+set the variable `running` to `True` before the while loop. First, we
+check if the variable `running` is `True` and then proceed to execute
+the corresponding *while-block*. After this block is executed, the
+condition is again checked which in this case is the `running`
+variable. If it is true, we execute the while-block again, else we
+continue to execute the optional else-block and then continue to the
+next statement.
+
+The `else` block is executed when the `while` loop condition becomes
+`False` - this may even be the first time that the condition is
+checked. If there is an `else` clause for a `while` loop, it is always
+executed unless you break out of the loop with a `break` statement.
+
+The `True` and `False` are called Boolean types and you can consider
+them to be equivalent to the value `1` and `0`respectively.
+
+Note for C/C++ Programmers
+
+:   Remember that you can have an `else` clause for the `while` loop.
+
+## The for loop ##
+
+The `for..in` statement is another looping statement which *iterates*
+over a sequence of objects i.e. go through each item in a sequence. We
+will see more about [sequences](#sequence) in detail in later
+chapters. What you need to know right now is that a sequence is just
+an ordered collection of items.
+
+Example (save as `for.py`):
+
+~~~python
+for i in range(1, 5):
+    print(i)
+else:
+    print('The for loop is over')
+~~~
+
+Output:
+
+~~~
+$ python3 for.py
+1
+2
+3
+4
+The for loop is over
+~~~
+
+How It Works:
+
+In this program, we are printing a *sequence* of numbers. We generate
+this sequence of numbers using the built-in `range` function.
+
+What we do here is supply it two numbers and `range` returns a
+sequence of numbers starting from the first number and up to the
+second number. For example, `range(1,5)` gives the sequence
+`[1, 2, 3, 4]`. By default, `range` takes a step count of 1. If we
+supply a third number to `range`, then that becomes the step
+count. For example, `range(1,5,2)` gives `[1,3]`. Remember that the
+range extends *up to* the second number i.e. it does **not** include
+the second number.
+
+Note that range() generates a sequence of numbers, but it will
+generate only one number at a time, when the for loop requests for the
+next item. If you want to see the full sequence of numbers
+immediately, use list(range()). Lists are explained in the
+[data structures chapter](#data-structures)].
+
+The `for` loop then iterates over this range - `for i in range(1,5)`
+is equivalent to `for i in [1, 2, 3, 4]` which is like assigning each
+number (or object) in the sequence to i, one at a time, and then
+executing the block of statements for each value of `i`.  In this
+case, we just print the value in the block of statements.
+
+Remember that the `else` part is optional. When included, it is always
+executed once after the `for` loop is over unless a [break](#break)
+statement is encountered.
+
+Remember that the `for..in` loop works for any sequence. Here, we have
+a list of numbers generated by the built-in `range` function, but in
+general we can use any kind of sequence of any kind of objects! We
+will explore this idea in detail in later chapters.
+
+Note for C/C++/Java/C# Programmers
+
+:   The Python `for` loop is radically different from the C/C++ `for`
+    loop. C# programmers will note that the `for` loop in Python is
+    similar to the `foreach` loop in C#. Java programmers will note that
+    the same is similar to `for (int i : IntArray)` in Java 1.5 .
+
+    In C/C++, if you want to write `for (int i = 0; i &lt; 5; i++)`,
+    then in Python you write just `for i in range(0,5)`. As you can
+    see, the `for` loop is simpler, more expressive and less error
+    prone in Python.
+
+## The break Statement ##
+
+The `break` statement is used to *break* out of a loop statement
+i.e. stop the execution of a looping statement, even if the loop
+condition has not become `False` or the sequence of items has not been
+completely iterated over.
+
+An important note is that if you *break* out of a `for`or `while`
+loop, any corresponding loop `else` block is **not** executed.
+
+Example (save as `break.py`):
+
+~~~python
+while True:
+    s = input('Enter something : ')
+    if s == 'quit':
+        break
+    print('Length of the string is', len(s))
+print('Done')
+~~~
+
+Output:
+
+~~~
+$ python3 break.py
+Enter something : Programming is fun
+Length of the string is 18
+Enter something : When the work is done
+Length of the string is 21
+Enter something : if you wanna make your work also fun:
+Length of the string is 37
+Enter something :       use Python!
+Length of the string is 12
+Enter something : quit
+Done
+~~~
+
+How It Works:
+
+In this program, we repeatedly take the user's input and print the
+length of each input each time. We are providing a special condition
+to stop the program by checking if the user input is `'quit'`. We stop
+the program by *breaking* out of the loop and reach the end of the
+program.
+
+The length of the input string can be found out using the built-in
+`len` function.
+
+Remember that the `break` statement can be used with the `for` loop as
+well.
+
+### Swaroop's Poetic Python ###
+
+The input I have used here is a mini poem I have written called
+*Swaroop's Poetic Python*:
+
+~~~
+Programming is fun
+When the work is done
+if you wanna make your work also fun:
+    use Python!
+~~~
+
+## The continue Statement ##
+
+The `continue` statement is used to tell Python to skip the rest of
+the statements in the current loop block and to *continue* to the next
+iteration of the loop.
+
+Example (save as `continue.py`):
+
+~~~python
+while True:
+    s = input('Enter something : ')
+    if s == 'quit':
+        break
+    if len(s) < 3:
+        print('Too small')
+        continue
+    print('Input is of sufficient length')
+    # Do other kinds of processing here...
+~~~
+
+Output:
+
+~~~
+$ python3 continue.py
+Enter something : a
+Too small
+Enter something : 12
+Too small
+Enter something : abc
+Input is of sufficient length
+Enter something : quit
+~~~
+
+How It Works:
+
+In this program, we accept input from the user, but we process the
+input string only if it is at least 3 characters long. So, we use the
+built-in `len` function to get the length and if the length is less
+than 3, we skip the rest of the statements in the block by using the
+`continue` statement. Otherwise, the rest of the statements in the
+loop are executed, doing any kind of processing we want to do here.
+
+Note that the `continue` statement works with the `for`loop as well.
+
+## Summary ##
+
+We have seen how to use the three control flow statements - `if`,
+`while` and `for` along with their associated `break` and `continue`
+statements. These are some of the most commonly used parts of Python
+and hence, becoming comfortable with them is essential.
+
+Next, we will see how to create and use functions.

09-functions.md

@@ -0,0 +1,524 @@
+# Functions #
+
+Functions are reusable pieces of programs. They allow you to give a
+name to a block of statements, allowing you to run that block using
+the specified name anywhere in your program and any number of
+times. This is known as *calling* the function. We have already used
+many built-in functions such as `len` and `range`.
+
+The function concept is probably *the* most important building block
+of any non-trivial software (in any programming language), so we will
+explore various aspects of functions in this chapter.
+
+Functions are defined using the `def` keyword. After this keyword
+comes an *identifier* name for the function, followed by a pair of
+parentheses which may enclose some names of variables, and by the
+final colon that ends the line. Next follows the block of statements
+that are part of this function. An example will show that this is
+actually very simple:
+
+Example (save as `function1.py`):
+
+~~~python
+def sayHello():
+    print('Hello World!') # block belonging to the function
+# End of function #
+
+sayHello() # call the function
+sayHello() # call the function again
+~~~
+
+Output:
+
+~~~
+$ python3 function1.py
+Hello World!
+Hello World!
+~~~
+
+How It Works:
+
+We define a function called `sayHello` using the syntax as explained
+above. This function takes no parameters and hence there are no
+variables declared in the parentheses. Parameters to functions are
+just input to the function so that we can pass in different values to
+it and get back corresponding results.
+
+Notice that we can call the same function twice which means we do not
+have to write the same code again.
+
+## Function Parameters ##
+
+A function can take parameters, which are values you supply to the
+function so that the function can *do* something utilising those
+values. These parameters are just like variables except that the
+values of these variables are defined when we call the function and
+are already assigned values when the function runs.
+
+Parameters are specified within the pair of parentheses in the
+function definition, separated by commas. When we call the function,
+we supply the values in the same way.  Note the terminology used - the
+names given in the function definition are called *parameters* whereas
+the values you supply in the function call are called *arguments*.
+
+Example (save as `func_param.py`):
+
+~~~python
+def printMax(a, b):
+    if a > b:
+        print(a, 'is maximum')
+    elif a == b:
+        print(a, 'is equal to', b)
+    else:
+        print(b, 'is maximum')
+
+printMax(3, 4) # directly give literal values
+
+x = 5
+y = 7
+
+printMax(x, y) # give variables as arguments
+~~~
+
+Output:
+
+~~~
+$ python3 func_param.py
+4 is maximum
+7 is maximum
+~~~
+
+How It Works:
+
+Here, we define a function called `printMax` that uses two parameters
+called `a` and `b`.  We find out the greater number using a simple
+`if..else` statement and then print the bigger number.
+
+The first time we call the function `printMax`, we directly supply the
+numbers as arguments. In the second case, we call the function with
+variables as arguments. `printMax(x, y)` causes the value of argument
+`x` to be assigned to parameter `a` and the value of argument `y` to
+be assigned to parameter `b`. The printMax function works the same way
+in both cases.
+
+## Local Variables ##
+
+When you declare variables inside a function definition, they are not
+related in any way to other variables with the same names used outside
+the function - i.e. variable names are *local* to the function. This
+is called the *scope* of the variable. All variables have the scope of
+the block they are declared in starting from the point of definition
+of the name.
+
+Example (save as `func_local.py`):
+
+~~~python
+x = 50
+
+def func(x):
+    print('x is', x)
+    x = 2
+    print('Changed local x to', x)
+
+func(x)
+print('x is still', x)
+~~~
+
+Output:
+
+~~~
+$ python3 func_local.py
+x is 50
+Changed local x to 2
+x is still 50
+~~~
+
+How It Works:
+
+The first time that we print the *value* of the name *x* with the
+first line in the function's body, Python uses the value of the
+parameter declared in the main block, above the function definition.
+
+Next, we assign the value `2` to `x`. The name `x` is local to our
+function.  So, when we change the value of `x` in the function, the
+`x` defined in the main block remains unaffected.
+
+With the last `print` function call, we display the value of `x` as
+defined in the main block, thereby confirming that it is actually
+unaffected by the local assignment within the previously called
+function.
+
+## Using The global Statement ##
+
+If you want to assign a value to a name defined at the top level of
+the program (i.e. not inside any kind of scope such as functions or
+classes), then you have to tell Python that the name is not local, but
+it is *global*. We do this using the `global` statement. It is
+impossible to assign a value to a variable defined outside a function
+without the `global` statement.
+
+You can use the values of such variables defined outside the function
+(assuming there is no variable with the same name within the
+function). However, this is not encouraged and should be avoided since
+it becomes unclear to the reader of the program as to where that
+variable's definition is. Using the `global` statement makes it amply
+clear that the variable is defined in an outermost block.
+
+Example (save as `func_global.py`):
+
+~~~python
+x = 50
+
+def func():
+    global x
+
+    print('x is', x)
+    x = 2
+    print('Changed global x to', x)
+
+func()
+print('Value of x is', x)
+~~~
+
+Output:
+
+~~~
+$ python3 func_global.py
+x is 50
+Changed global x to 2
+Value of x is 2
+~~~
+
+How It Works:
+
+The `global` statement is used to declare that `x` is a global
+variable - hence, when we assign a value to `x` inside the function,
+that change is reflected when we use the value of `x` in the main
+block.
+
+You can specify more than one global variable using the same `global`
+statement e.g. `global x, y, z`.
+
+## Default Argument Values ##
+
+For some functions, you may want to make some parameters *optional*
+and use default values in case the user does not want to provide
+values for them. This is done with the help of default argument
+values. You can specify default argument values for parameters by
+appending to the parameter name in the function definition the
+assignment operator (`=`) followed by the default value.
+
+Note that the default argument value should be a constant. More
+precisely, the default argument value should be immutable - this is
+explained in detail in later chapters. For now, just remember this.
+
+Example (save as `func_default.py`):
+
+~~~python
+def say(message, times = 1):
+    print(message * times)
+
+say('Hello')
+say('World', 5)
+~~~
+
+Output:
+
+~~~
+$ python3 func_default.py
+Hello
+WorldWorldWorldWorldWorld
+~~~
+
+How It Works:
+
+The function named `say` is used to print a string as many times as
+specified. If we don't supply a value, then by default, the string is
+printed just once. We achieve this by specifying a default argument
+value of `1` to the parameter `times`.
+
+In the first usage of `say`, we supply only the string and it prints
+the string once. In the second usage of `say`, we supply both the
+string and an argument `5` stating that we want to *say* the string
+message 5 times.
+
+Important
+
+:   Only those parameters which are at the end of the parameter list can
+    be given default argument values i.e. you cannot have a parameter with
+    a default argument value preceding a parameter without a default
+    argument value in the function's parameter list.
+
+    This is because the values are assigned to the parameters by
+    position. For example,`def func(a, b=5)` is valid, but `def
+    func(a=5, b)` is *not valid*.
+
+## Keyword Arguments ##
+
+If you have some functions with many parameters and you want to
+specify only some of them, then you can give values for such
+parameters by naming them - this is called *keyword arguments* - we
+use the name (keyword) instead of the position (which we have been
+using all along) to specify the arguments to the function.
+
+There are two *advantages* - one, using the function is easier since
+we do not need to worry about the order of the arguments. Two, we can
+give values to only those parameters to which we want to, provided
+that the other parameters have default argument values.
+
+Example (save as `func_key.py`):
+
+~~~python
+def func(a, b=5, c=10):
+    print('a is', a, 'and b is', b, 'and c is', c)
+
+func(3, 7)
+func(25, c=24)
+func(c=50, a=100)
+~~~
+
+Output:
+
+~~~
+$ python3 func_key.py
+a is 3 and b is 7 and c is 10
+a is 25 and b is 5 and c is 24
+a is 100 and b is 5 and c is 50
+~~~
+
+How It Works:
+
+The function named `func` has one parameter without a default argument
+value, followed by two parameters with default argument values.
+
+In the first usage, `func(3, 7)`, the parameter `a` gets the value
+`3`, the parameter `b` gets the value `7` and `c` gets the default
+value of `10`.
+
+In the second usage `func(25, c=24)`, the variable `a` gets the value
+of 25 due to the position of the argument. Then, the parameter `c`
+gets the value of `24` due to naming i.e. keyword arguments. The
+variable `b` gets the default value of `5`.
+
+In the third usage `func(c=50, a=100)`, we use keyword arguments for
+all specified values. Notice that we are specifying the value for
+parameter `c` before that for `a` even though `a` is defined before
+`c` in the function definition.
+
+## VarArgs parameters ##
+
+Sometimes you might want to define a function that can take *any*
+number of parameters, this can be achieved by using the stars (save as
+`total.py`):
+
+~~~python
+def total(initial=5, *numbers, **keywords):
+    count = initial
+    for number in numbers:
+        count += number
+    for key in keywords:
+        count += keywords[key]
+    return count
+
+print(total(10, 1, 2, 3, vegetables=50, fruits=100))
+~~~
+
+Output:
+
+~~~
+$ python3 total.py
+166
+~~~
+
+How It Works:
+
+When we declare a starred parameter such as `*param`, then all the
+positional arguments from that point till the end are collected as a
+tuple called 'param'.
+
+Similarly, when we declare a double-starred parameter such as
+`**param`, then all the keyword arguments from that point till the end
+are collected as a dictionary called 'param'.
+
+We will explore tuples and dictionaries in a
+[later chapter](#data-structures).
+
+## Keyword-only Parameters ##
+
+If we want to specify certain keyword parameters to be available as
+keyword-only and *not* as positional arguments, they can be declared
+after a starred parameter (save as `keyword_only.py`):
+
+~~~python
+def total(initial=5, *numbers, extra_number):
+    count = initial
+    for number in numbers:
+        count += number
+    count += extra_number
+    print(count)
+
+total(10, 1, 2, 3, extra_number=50)
+total(10, 1, 2, 3)
+# Raises error because we have not supplied a default argument value for 'extra_number' #
+~~~
+
+Output:
+
+~~~
+$ python3 keyword_only.py
+66
+Traceback (most recent call last):
+  File "keyword_only.py", line 12, in <module>
+total(10, 1, 2, 3)
+TypeError: total() needs keyword-only argument extra_number
+~~~
+
+How It Works:
+
+Declaring parameters after a starred parameter results in keyword-only
+arguments. If these arguments are not supplied a default value, then
+calls to the function will raise an error if the keyword argument is
+not supplied, as seen above.
+
+Notice the use of `+=` which is a shortcut operator, so instead of
+saying `x = x + y`, you can say `x += y`.
+
+If you want to have keyword-only arguments but have no need for a
+starred parameter, then simply use an empty star without using any
+name such as `def total(initial=5, *, extra_number)`.
+
+## The return Statement ##
+
+The `return` statement is used to *return* from a function i.e. break
+out of the function. We can optionally *return a value* from the
+function as well.
+
+Example (save as `func_return.py`):
+
+~~~python
+def maximum(x, y):
+    if x > y:
+        return x
+    elif x == y:
+        return 'The numbers are equal'
+    else:
+        return y
+
+print(maximum(2, 3))
+~~~
+
+Output:
+
+~~~
+$ python3 func_return.py
+3
+~~~
+
+How It Works:
+
+The `maximum` function returns the maximum of the parameters, in this
+case the numbers supplied to the function. It uses a simple `if..else`
+statement to find the greater value and then *returns* that value.
+
+Note that a `return` statement without a value is equivalent to
+`return None`. `None` is a special type in Python that represents
+nothingness. For example, it is used to indicate that a variable has
+no value if it has a value of `None`.
+
+Every function implicitly contains a `return None` statement at the
+end unless you have written your own `return` statement. You can see
+this by running `print(someFunction())` where the function
+`someFunction` does not use the `return` statement such as:
+
+~~~python
+def someFunction():
+    pass
+~~~
+
+The `pass` statement is used in Python to indicate an empty block of
+statements.
+
+Note
+
+:   There is a built-in function called `max` that already implements
+    the 'find maximum' functionality, so use this built-in function
+    whenever possible.
+
+## DocStrings ##
+
+Python has a nifty feature called *documentation strings*, usually
+referred to by its shorter name *docstrings*. DocStrings are an
+important tool that you should make use of since it helps to document
+the program better and makes it easier to understand. Amazingly, we
+can even get the docstring back from, say a function, when the program
+is actually running!
+
+Example (save as `func_doc.py`):
+
+~~~python
+def printMax(x, y):
+    '''Prints the maximum of two numbers.
+
+    The two values must be integers.'''
+    x = int(x) # convert to integers, if possible
+    y = int(y)
+
+    if x > y:
+        print(x, 'is maximum')
+    else:
+        print(y, 'is maximum')
+
+printMax(3, 5)
+print(printMax.__doc__)
+~~~
+
+Output:
+
+~~~
+$ python3 func_doc.py
+5 is maximum
+Prints the maximum of two numbers.
+
+        The two values must be integers.
+~~~
+
+How It Works:
+
+A string on the first logical line of a function is the *docstring*
+for that function. Note that DocStrings also apply to
+[Modules](#modules) and [classes](#object-oriented-programming) which
+we will learn about in the respective chapters.
+
+The convention followed for a docstring is a multi-line string where
+the first line starts with a capital letter and ends with a dot. Then
+the second line is blank followed by any detailed explanation starting
+from the third line. You are *strongly advised* to follow this
+convention for all your docstrings for all your non-trivial functions.
+
+We can access the docstring of the `printMax` function using the
+`__doc__` (notice the *double underscores*) attribute (name belonging
+to) of the function. Just remember that Python treats *everything* as
+an object and this includes functions. We'll learn more about objects
+in the chapter on [classes](#object-oriented-programming).
+
+If you have used `help()` in Python, then you have already seen the
+usage of docstrings! What it does is just fetch the `__doc__`
+attribute of that function and displays it in a neat manner for
+you. You can try it out on the function above - just include
+`help(printMax)` in your program. Remember to press the `q` key to
+exit `help`.
+
+Automated tools can retrieve the documentation from your program in
+this manner. Therefore, I *strongly recommend* that you use docstrings
+for any non-trivial function that you write. The `pydoc` command that
+comes with your Python distribution works similarly to `help()` using
+docstrings.
+
+## Summary ##
+
+We have seen so many aspects of functions but note that we still
+haven't covered all aspects of them. However, we have already covered
+most of what you'll use regarding Python functions on an everyday
+basis.
+
+Next, we will see how to use as well as create Python modules.

09-functions.pd

@@ -1,377 +0,0 @@
-# Functions
-
-Functions are reusable pieces of programs. They allow you to give a name to a block of statements, allowing you to run that block using the specified name anywhere in your program and any number of times. This is known as *calling* the function. We have already used many built-in functions such as `len` and `range`.
-
-The function concept is probably *the* most important building block of any non-trivial software (in any programming language), so we will explore various aspects of functions in this chapter.
-
-Functions are defined using the `def` keyword. After this keyword comes an *identifier* name for the function, followed by a pair of parentheses which may enclose some names of variables, and by the final colon that ends the line. Next follows the block of statements that are part of this function. An example will show that this is actually very simple:
-
-Example (save as `function1.py`):
-
-~~~python
-def sayHello():
-    print('Hello World!') # block belonging to the function
-# End of function
-
-sayHello() # call the function
-sayHello() # call the function again
-~~~
-
-Output:
-
-~~~
-$ python3 function1.py
-Hello World!
-Hello World!
-~~~
-
-How It Works:
-
-We define a function called `sayHello` using the syntax as explained above. This function takes no parameters and hence there are no variables declared in the parentheses. Parameters to functions are just input to the function so that we can pass in different values to it and get back corresponding results.
-
-Notice that we can call the same function twice which means we do not have to write the same code again.
-
-## Function Parameters 
-
-A function can take parameters, which are values you supply to the function so that the function can *do* something utilising those values. These parameters are just like variables except that the values of these variables are defined when we call the function and are already assigned values when the function runs.
-
-Parameters are specified within the pair of parentheses in the function definition, separated by commas. When we call the function, we supply the values in the same way.  Note the terminology used - the names given in the function definition are called *parameters* whereas the values you supply in the function call are called *arguments*.
-
-Example (save as `func_param.py`):
-
-~~~python
-def printMax(a, b):
-    if a > b:
-        print(a, 'is maximum')
-    elif a == b:
-        print(a, 'is equal to', b)
-    else:
-        print(b, 'is maximum')
-
-printMax(3, 4) # directly give literal values
-
-x = 5
-y = 7
-
-printMax(x, y) # give variables as arguments
-~~~
-
-Output:
-
-~~~
-$ python3 func_param.py
-4 is maximum
-7 is maximum
-~~~
-
-How It Works:
-
-Here, we define a function called `printMax` that uses two parameters called `a` and `b`.  We find out the greater number using a simple `if..else` statement and then print the bigger number.
-
-The first time we call the function `printMax`, we directly supply the numbers as arguments. In the second case, we call the function with variables as arguments. `printMax(x, y)` causes the value of argument `x` to be assigned to parameter `a` and the value of argument `y` to be assigned to parameter `b`. The printMax function works the same way in both cases.
-
-## Local Variables 
-
-When you declare variables inside a function definition, they are not related in any way to other variables with the same names used outside the function - i.e. variable names are *local* to the function. This is called the *scope* of the variable. All variables have the scope of the block they are declared in starting from the point of definition of the name.
-
-Example (save as `func_local.py`):
-
-~~~python
-x = 50
-
-def func(x):
-    print('x is', x)
-    x = 2
-    print('Changed local x to', x)
-
-func(x)
-print('x is still', x)
-~~~
-
-Output:
-
-~~~
-$ python3 func_local.py
-x is 50
-Changed local x to 2
-x is still 50
-~~~
-
-How It Works:
-
-The first time that we print the *value* of the name *x* with the first line in the function's body, Python uses the value of the parameter declared in the main block, above the function definition.
-
-Next, we assign the value `2` to `x`. The name `x` is local to our function.  So, when we change the value of `x` in the function, the `x` defined in the main block remains unaffected.
-
-With the last `print` function call, we display the value of `x` as defined in the main block, thereby confirming that it is actually unaffected by the local assignment within the previously called function.
-
-## Using The global Statement
-
-If you want to assign a value to a name defined at the top level of the program (i.e. not inside any kind of scope such as functions or classes), then you have to tell Python that the name is not local, but it is *global*. We do this using the `global` statement. It is impossible to assign a value to a variable defined outside a function without the `global` statement.
-
-You can use the values of such variables defined outside the function (assuming there is no variable with the same name within the function). However, this is not encouraged and should be avoided since it becomes unclear to the reader of the program as to where that variable's definition is. Using the `global` statement makes it amply clear that the variable is defined in an outermost block.
-
-Example (save as `func_global.py`):
-
-~~~python
-x = 50
-
-def func():
-    global x
-
-    print('x is', x)
-    x = 2
-    print('Changed global x to', x)
-
-func()
-print('Value of x is', x)
-~~~
-
-Output:
-
-~~~
-$ python3 func_global.py
-x is 50
-Changed global x to 2
-Value of x is 2
-~~~
-
-How It Works:
-
-The `global` statement is used to declare that `x` is a global variable - hence, when we assign a value to `x` inside the function, that change is reflected when we use the value of `x` in the main block.
-
-You can specify more than one global variable using the same `global` statement e.g. `global x, y, z`.
-
-## Default Argument Values 
-
-For some functions, you may want to make some parameters *optional* and use default values in case the user does not want to provide values for them. This is done with the help of default argument values. You can specify default argument values for parameters by appending to the parameter name in the function definition the assignment operator (`=`) followed by the default value.
-
-Note that the default argument value should be a constant. More precisely, the default argument value should be immutable - this is explained in detail in later chapters. For now, just remember this.
-
-Example (save as `func_default.py`):
-
-~~~python
-def say(message, times = 1):
-    print(message * times)
-
-say('Hello')
-say('World', 5)
-~~~
-
-Output:
-
-~~~
-$ python3 func_default.py
-Hello
-WorldWorldWorldWorldWorld
-~~~
-
-How It Works:
-
-The function named `say` is used to print a string as many times as specified. If we don't supply a value, then by default, the string is printed just once. We achieve this by specifying a default argument value of `1` to the parameter `times`.
-
-In the first usage of `say`, we supply only the string and it prints the string once. In the second usage of `say`, we supply both the string and an argument `5` stating that we want to *say* the string message 5 times.
-
-Important
-
-:   Only those parameters which are at the end of the parameter list can be given default argument values i.e. you cannot have a parameter with a default argument value preceding a parameter without a default argument value in the function's parameter list.
-
-    This is because the values are assigned to the parameters by position. For example,`def func(a, b=5)` is valid, but `def func(a=5, b)` is *not valid*.
-
-## Keyword Arguments 
-
-If you have some functions with many parameters and you want to specify only some of them, then you can give values for such parameters by naming them - this is called *keyword arguments* - we use the name (keyword) instead of the position (which we have been using all along) to specify the arguments to the function.
-
-There are two *advantages* - one, using the function is easier since we do not need to worry about the order of the arguments. Two, we can give values to only those parameters to which we want to, provided that the other parameters have default argument values.
-
-Example (save as `func_key.py`):
-
-~~~python
-def func(a, b=5, c=10):
-    print('a is', a, 'and b is', b, 'and c is', c)
-
-func(3, 7)
-func(25, c=24)
-func(c=50, a=100)
-~~~
-
-Output:
-
-~~~
-$ python3 func_key.py
-a is 3 and b is 7 and c is 10
-a is 25 and b is 5 and c is 24
-a is 100 and b is 5 and c is 50
-~~~
-
-How It Works:
-
-The function named `func` has one parameter without a default argument value, followed by two parameters with default argument values.
-
-In the first usage, `func(3, 7)`, the parameter `a` gets the value `3`, the parameter `b` gets the value `7` and `c` gets the default value of `10`.
-
-In the second usage `func(25, c=24)`, the variable `a` gets the value of 25 due to the position of the argument. Then, the parameter `c` gets the value of `24` due to naming i.e. keyword arguments. The variable `b` gets the default value of `5`.
-
-In the third usage `func(c=50, a=100)`, we use keyword arguments for all specified values. Notice that we are specifying the value for parameter `c` before that for `a` even though `a` is defined before `c` in the function definition.
-
-## VarArgs parameters
-
-Sometimes you might want to define a function that can take *any* number of parameters, this can be achieved by using the stars (save as `total.py`):
-
-~~~python
-def total(initial=5, *numbers, **keywords):
-    count = initial
-    for number in numbers:
-        count += number
-    for key in keywords:
-        count += keywords[key]
-    return count
-
-print(total(10, 1, 2, 3, vegetables=50, fruits=100))
-~~~
-
-Output:
-
-~~~
-$ python3 total.py
-166
-~~~
-
-How It Works:
-
-When we declare a starred parameter such as `*param`, then all the positional arguments from that point till the end are collected as a tuple called 'param'.
-
-Similarly, when we declare a double-starred parameter such as `**param`, then all the keyword arguments from that point till the end are collected as a dictionary called 'param'.
-
-We will explore tuples and dictionaries in a [later chapter](#data-structures).
-
-## Keyword-only Parameters 
-
-If we want to specify certain keyword parameters to be available as keyword-only and *not* as positional arguments, they can be declared after a starred parameter (save as `keyword_only.py`):
-
-~~~python
-def total(initial=5, *numbers, extra_number):
-    count = initial
-    for number in numbers:
-        count += number
-    count += extra_number
-    print(count)
-
-total(10, 1, 2, 3, extra_number=50)
-total(10, 1, 2, 3)
-# Raises error because we have not supplied a default argument value for 'extra_number'
-~~~
-
-Output:
-
-~~~
-$ python3 keyword_only.py
-66
-Traceback (most recent call last):
-  File "keyword_only.py", line 12, in <module>
-total(10, 1, 2, 3)
-TypeError: total() needs keyword-only argument extra_number
-~~~
-
-How It Works:
-
-Declaring parameters after a starred parameter results in keyword-only arguments. If these arguments are not supplied a default value, then calls to the function will raise an error if the keyword argument is not supplied, as seen above.
-
-Notice the use of `+=` which is a shortcut operator, so instead of saying `x = x + y`, you can say `x += y`.
-
-If you want to have keyword-only arguments but have no need for a starred parameter, then simply use an empty star without using any name such as `def total(initial=5, *, extra_number)`.
-
-## The return Statement 
-
-The `return` statement is used to *return* from a function i.e. break out of the function. We can optionally *return a value* from the function as well.
-
-Example (save as `func_return.py`):
-
-~~~python
-def maximum(x, y):
-    if x > y:
-        return x
-    elif x == y:
-        return 'The numbers are equal'
-    else:
-        return y
-
-print(maximum(2, 3))
-~~~
-
-Output:
-
-~~~
-$ python3 func_return.py
-3
-~~~
-
-How It Works:
-
-The `maximum` function returns the maximum of the parameters, in this case the numbers supplied to the function. It uses a simple `if..else` statement to find the greater value and then *returns* that value.
-
-Note that a `return` statement without a value is equivalent to `return None`. `None` is a special type in Python that represents nothingness. For example, it is used to indicate that a variable has no value if it has a value of `None`.
-
-Every function implicitly contains a `return None` statement at the end unless you have written your own `return` statement. You can see this by running `print(someFunction())` where the function `someFunction` does not use the `return` statement such as:
-
-~~~python
-def someFunction():
-    pass
-~~~
-
-The `pass` statement is used in Python to indicate an empty block of statements.
-
-Note
-
-:   There is a built-in function called `max` that already implements the 'find maximum' functionality, so use this built-in function whenever possible.
-
-## DocStrings 
-
-Python has a nifty feature called *documentation strings*, usually referred to by its shorter name *docstrings*. DocStrings are an important tool that you should make use of since it helps to document the program better and makes it easier to understand. Amazingly, we can even get the docstring back from, say a function, when the program is actually running!
-
-Example (save as `func_doc.py`):
-
-~~~python
-def printMax(x, y):
-    '''Prints the maximum of two numbers.
-
-    The two values must be integers.'''
-    x = int(x) # convert to integers, if possible
-    y = int(y)
-
-    if x > y:
-        print(x, 'is maximum')
-    else:
-        print(y, 'is maximum')
-
-printMax(3, 5)
-print(printMax.__doc__)
-~~~
-
-Output:
-
-~~~
-$ python3 func_doc.py
-5 is maximum
-Prints the maximum of two numbers.
-
-        The two values must be integers.
-~~~
-
-How It Works:
-
-A string on the first logical line of a function is the *docstring* for that function. Note that DocStrings also apply to [Modules](#modules) and [classes](#object-oriented-programming) which we will learn about in the respective chapters.
-
-The convention followed for a docstring is a multi-line string where the first line starts with a capital letter and ends with a dot. Then the second line is blank followed by any detailed explanation starting from the third line. You are *strongly advised* to follow this convention for all your docstrings for all your non-trivial functions.
-
-We can access the docstring of the `printMax` function using the `__doc__` (notice the *double underscores*) attribute (name belonging to) of the function. Just remember that Python treats *everything* as an object and this includes functions. We'll learn more about objects in the chapter on [classes](#object-oriented-programming).
-
-If you have used `help()` in Python, then you have already seen the usage of docstrings! What it does is just fetch the `__doc__` attribute of that function and displays it in a neat manner for you. You can try it out on the function above - just include `help(printMax)` in your program. Remember to press the `q` key to exit `help`.
-
-Automated tools can retrieve the documentation from your program in this manner. Therefore, I *strongly recommend* that you use docstrings for any non-trivial function that you write. The `pydoc` command that comes with your Python distribution works similarly to `help()` using docstrings.
-
-## Summary 
-
-We have seen so many aspects of functions but note that we still haven't covered all aspects of them. However, we have already covered most of what you'll use regarding Python functions on an everyday basis.
-
-Next, we will see how to use as well as create Python modules.

10-modules.md

@@ -0,0 +1,390 @@
+# Modules #
+
+You have seen how you can reuse code in your program by defining
+functions once. What if you wanted to reuse a number of functions in
+other programs that you write? As you might have guessed, the answer
+is modules.
+
+There are various methods of writing modules, but the simplest way is
+to create a file with a `.py` extension that contains functions and
+variables.
+
+Another method is to write the modules in the native language in which
+the Python interpreter itself was written. For example, you can write
+modules in the
+[C programming language](http://docs.python.org/3/extending/)
+and when compiled, they can be used from your Python code when using
+the standard Python interpreter.
+
+A module can be *imported* by another program to make use of its
+functionality. This is how we can use the Python standard library as
+well. First, we will see how to use the standard library modules.
+
+Example (save as `using_sys.py`):
+
+~~~python
+import sys
+
+print('The command line arguments are:')
+for i in sys.argv:
+    print(i)
+
+print('\n\nThe PYTHONPATH is', sys.path, '\n')
+~~~
+
+Output:
+
+~~~
+$ python3 using_sys.py we are arguments
+The command line arguments are:
+using_sys.py
+we
+are
+arguments
+
+
+The PYTHONPATH is ['/Users/swaroop/byte_of_python',
+'/usr/local/Cellar/python3/3.3.2/Frameworks/Python.framework/Versions/3.3/lib/python3.3',
+'/usr/local/Cellar/python3/3.3.2/Frameworks/Python.framework/Versions/3.3/lib/python3.3/plat-darwin',
+'/usr/local/Cellar/python3/3.3.2/Frameworks/Python.framework/Versions/3.3/lib/python3.3/lib-dynload',
+'/usr/local/lib/python3.3/site-packages']
+~~~
+
+How It Works:
+
+First, we *import* the `sys` module using the `import`
+statement. Basically, this translates to us telling Python that we
+want to use this module. The `sys` module contains functionality
+related to the Python interpreter and its environment i.e. the
+*sys*tem.
+
+When Python executes the `import sys` statement, it looks for the
+`sys` module. In this case, it is one of the built-in modules, and
+hence Python knows where to find it.
+
+If it was not a compiled module i.e. a module written in Python, then
+the Python interpreter will search for it in the directories listed in
+its `sys.path` variable. If the module is found, then the statements
+in the body of that module are run and the module is made *available*
+for you to use. Note that the initialization is done only the *first*
+time that we import a module.
+
+The `argv` variable in the `sys` module is accessed using the dotted
+notation i.e. `sys.argv`. It clearly indicates that this name is part
+of the `sys` module. Another advantage of this approach is that the
+name does not clash with any `argv` variable used in your program.
+
+The `sys.argv` variable is a *list* of strings (lists are explained in
+detail in a [later chapter](#data-structures)). Specifically, the
+`sys.argv` contains the list of *command line arguments* i.e. the
+arguments passed to your program using the command line.
+
+If you are using an IDE to write and run these programs, look for a
+way to specify command line arguments to the program in the menus.
+
+Here, when we execute `python using_sys.py we are arguments`, we run
+the module `using_sys.py` with the `python` command and the other
+things that follow are arguments passed to the program. Python stores
+the command line arguments in the `sys.argv` variable for us to use.
+
+Remember, the name of the script running is always the first argument
+in the `sys.argv` list. So, in this case we will have `'using_sys.py'`
+as `sys.argv[0]`, `'we'` as `sys.argv[1]`, `'are'` as `sys.argv[2]`
+and `'arguments'` as `sys.argv[3]`. Notice that Python starts counting
+from 0 and not 1.
+
+The `sys.path` contains the list of directory names where modules are
+imported from. Observe that the first string in `sys.path` is empty -
+this empty string indicates that the current directory is also part of
+the `sys.path` which is same as the `PYTHONPATH` environment
+variable. This means that you can directly import modules located in
+the current directory. Otherwise, you will have to place your module
+in one of the directories listed in `sys.path`.
+
+Note that the current directory is the directory from which the
+program is launched. Run `import os; print(os.getcwd())` to find out
+the current directory of your program.
+
+## Byte-compiled .pyc files ##
+
+Importing a module is a relatively costly affair, so Python does some
+tricks to make it faster. One way is to create *byte-compiled* files
+with the extension `.pyc` which is an intermediate form that Python
+transforms the program into (remember the
+[introduction section](#introduction) on how Python works?). This
+`.pyc` file is useful when you import the module the next time from a
+different program - it will be much faster since a portion of the
+processing required in importing a module is already done. Also, these
+byte-compiled files are platform-independent.
+
+Note
+
+:   These `.pyc` files are usually created in the same directory as
+    the corresponding `.py` files. If Python does not have permission
+    to write to files in that directory, then the `.pyc` files will
+    not be created.
+
+## The from ... import statement ##
+
+If you want to directly import the `argv` variable into your program
+(to avoid typing the `sys.` everytime for it), then you can use the
+`from sys import argv` statement.
+
+In general, you *should avoid* using this statement and use the
+`import` statement instead since your program will avoid name clashes
+and will be more readable.
+
+Example:
+
+~~~python
+from math import sqrt
+print("Square root of 16 is", sqrt(16))
+~~~
+
+## A module's __name__ ##
+
+Every module has a name and statements in a module can find out the
+name of their module. This is handy for the particular purpose of
+figuring out whether the module is being run standalone or being
+imported. As mentioned previously, when a module is imported for the
+first time, the code it contains gets executed. We can use this to
+make the module behave in different ways depending on whether it is
+being used by itself or being imported from another module. This can
+be achieved using the `__name__` attribute of the module.
+
+Example (save as `using_name.py`):
+
+~~~python
+if __name__ == '__main__':
+    print('This program is being run by itself')
+else:
+    print('I am being imported from another module')
+~~~
+
+Output:
+
+~~~
+$ python3 using_name.py
+This program is being run by itself
+
+$ python3
+>>> import using_name
+I am being imported from another module
+>>>
+~~~
+
+How It Works:
+
+Every Python module has its `__name__` defined. If this is
+`'__main__'`, that implies that the module is being run standalone by
+the user and we can take appropriate actions.
+
+## Making Your Own Modules ##
+
+Creating your own modules is easy, you've been doing it all along!
+This is because every Python program is also a module. You just have
+to make sure it has a `.py` extension. The following example should
+make it clear.
+
+Example (save as `mymodule.py`):
+
+~~~python
+def sayhi():
+    print('Hi, this is mymodule speaking.')
+
+__version__ = '0.1'
+~~~
+
+The above was a sample *module*. As you can see, there is nothing
+particularly special about it compared to our usual Python program. We
+will next see how to use this module in our other Python programs.
+
+Remember that the module should be placed either in the same directory
+as the program from which we import it, or in one of the directories
+listed in `sys.path`.
+
+Another module (save as `mymodule_demo.py`):
+
+~~~python
+import mymodule
+
+mymodule.sayhi()
+print ('Version', mymodule.__version__)
+~~~
+
+Output:
+
+~~~
+$ python3 mymodule_demo.py
+Hi, this is mymodule speaking.
+Version 0.1
+~~~
+
+How It Works:
+
+Notice that we use the same dotted notation to access members of the
+module. Python makes good reuse of the same notation to give the
+distinctive 'Pythonic' feel to it so that we don't have to keep
+learning new ways to do things.
+
+Here is a version utilising the `from..import` syntax (save as
+`mymodule_demo2.py`):
+
+~~~python
+from mymodule import sayhi, __version__
+
+sayhi()
+print('Version', __version__)
+~~~
+
+The output of `mymodule_demo2.py` is same as the output of
+`mymodule_demo.py`.
+
+Notice that if there was already a `__version__` name declared in the
+module that imports mymodule, there would be a clash. This is also
+likely because it is common practice for each module to declare it's
+version number using this name. Hence, it is always recommended to
+prefer the `import` statement even though it might make your program a
+little longer.
+
+You could also use:
+
+~~~python
+from mymodule import *
+~~~
+
+This will import all public names such as `sayhi` but would not import
+`__version__` because it starts with double underscores.
+
+Zen of Python
+
+:   One of Python's guiding principles is that "Explicit is better
+    than Implicit". Run `import this` to learn more and see
+    [this StackOverflow discussion](http://stackoverflow.com/questions/228181/zen-of-python)
+    which lists examples for each of the principles.
+
+## The dir function ##
+
+You can use the built-in `dir` function to list the identifiers that
+an object defines. For example, for a module, the identifiers include
+the functions, classes and variables defined in that module.
+
+When you supply a module name to the`dir()` function, it returns the
+list of the names defined in that module. When no argument is applied
+to it, it returns the list of names defined in the current module.
+
+Example:
+
+~~~
+$ python3
+
+>>> import sys # get list of attributes, in this case, for the sys module
+
+>>> dir(sys)
+['__displayhook__', '__doc__', '__excepthook__', '__name__', '__package__', '__s
+tderr__', '__stdin__', '__stdout__', '_clear_type_cache', '_compact_freelists',
+'_current_frames', '_getframe', 'api_version', 'argv', 'builtin_module_names', '
+byteorder', 'call_tracing', 'callstats', 'copyright', 'displayhook', 'dllhandle'
+, 'dont_write_bytecode', 'exc_info', 'excepthook', 'exec_prefix', 'executable',
+'exit', 'flags', 'float_info', 'getcheckinterval', 'getdefaultencoding', 'getfil
+esystemencoding', 'getprofile', 'getrecursionlimit', 'getrefcount', 'getsizeof',
+'gettrace', 'getwindowsversion', 'hexversion', 'intern', 'maxsize', 'maxunicode
+', 'meta_path', 'modules', 'path', 'path_hooks', 'path_importer_cache', 'platfor
+m', 'prefix', 'ps1', 'ps2', 'setcheckinterval', 'setprofile', 'setrecursionlimit
+', 'settrace', 'stderr', 'stdin', 'stdout', 'subversion', 'version', 'version_in
+fo', 'warnoptions', 'winver']
+
+>>> dir() # get list of attributes for current module
+['__builtins__', '__doc__', '__name__', '__package__', 'sys']
+
+>>> a = 5 # create a new variable 'a'
+
+>>> dir()
+['__builtins__', '__doc__', '__name__', '__package__', 'a', 'sys']
+
+>>> del a # delete/remove a name
+
+>>> dir()
+['__builtins__', '__doc__', '__name__', '__package__', 'sys']
+
+>>>
+~~~
+
+How It Works:
+
+First, we see the usage of `dir` on the imported `sys` module. We can
+see the huge list of attributes that it contains.
+
+Next, we use the `dir` function without passing parameters to it. By
+default, it returns the list of attributes for the current
+module. Notice that the list of imported modules is also part of this
+list.
+
+In order to observe the `dir` in action, we define a new variable `a`
+and assign it a value and then check `dir`and we observe that there is
+an additional value in the list of the same name. We remove the
+variable/attribute of the current module using the `del` statement and
+the change is reflected again in the output of the `dir` function.
+
+A note on `del` - this statement is used to *delete* a variable/name
+and after the statement has run, in this case `del a`, you can no
+longer access the variable `a` - it is as if it never existed before
+at all.
+
+Note that the `dir()` function works on *any* object. For example, run
+`dir('print')` to learn about the attributes of the print function, or
+`dir(str)` for the attributes of the str class.
+
+There is also a
+[`vars()`](http://docs.python.org/3/library/functions.html#vars)
+function which can potentially give you the attributes and their
+values, but it will not work for all cases.
+
+## Packages ##
+
+By now, you must have started observing the hierarchy of organizing
+your programs. Variables usually go inside functions. Functions and
+global variables usually go inside modules. What if you wanted to
+organize modules? That's where packages come into the picture.
+
+Packages are just folders of modules with a special `__init__.py` file
+that indicates to Python that this folder is special because it
+contains Python modules.
+
+Let's say you want to create a package called 'world' with subpackages
+'asia', 'africa', etc. and these subpackages in turn contain modules
+like 'india', 'madagascar', etc.
+
+This is how you would structure the folders:
+
+~~~
+- <some folder present in the sys.path>/
+    - world/
+        - __init__.py
+        - asia/
+            - __init__.py
+            - india/
+                - __init__.py
+                - foo.py
+        - africa/
+            - __init__.py
+            - madagascar/
+                - __init__.py
+                - bar.py
+~~~
+
+Packages are just a convenience to hierarchically organize
+modules. You will see many instances of this in the
+[standard library](#standard-library).
+
+## Summary ##
+
+Just like functions are reusable parts of programs, modules are
+reusable programs. Packages are another hierarchy to organize
+modules. The standard library that comes with Python is an example of
+such a set of packages and modules.
+
+We have seen how to use these modules and create our own modules.
+
+Next, we will learn about some interesting concepts called data
+structures.

11-data-structures.md

@@ -0,0 +1,601 @@
+# Data Structures #
+
+Data structures are basically just that - they are *structures* which
+can hold some *data* together. In other words, they are used to store
+a collection of related data.
+
+There are four built-in data structures in Python - list, tuple,
+dictionary and set. We will see how to use each of them and how they
+make life easier for us.
+
+## List ##
+
+A `list` is a data structure that holds an ordered collection of items
+i.e. you can store a *sequence* of items in a list. This is easy to
+imagine if you can think of a shopping list where you have a list of
+items to buy, except that you probably have each item on a separate
+line in your shopping list whereas in Python you put commas in between
+them.
+
+The list of items should be enclosed in square brackets so that Python
+understands that you are specifying a list. Once you have created a
+list, you can add, remove or search for items in the list. Since we
+can add and remove items, we say that a list is a *mutable* data type
+i.e. this type can be altered.
+
+### Quick Introduction To Objects And Classes ###
+
+Although I've been generally delaying the discussion of objects and
+classes till now, a little explanation is needed right now so that you
+can understand lists better. We will explore this topic in detail in a
+[later chapter](#object-oriented-programming).
+
+A list is an example of usage of objects and classes. When we use a
+variable `i` and assign a value to it, say integer `5` to it, you can
+think of it as creating an **object** (i.e. instance) `i` of **class**
+(i.e. type) `int`. In fact, you can read `help(int)` to understand
+this better.
+
+A class can also have **methods** i.e. functions defined for use with
+respect to that class only. You can use these pieces of functionality
+only when you have an object of that class. For example, Python
+provides an `append` method for the `list` class which allows you to
+add an item to the end of the list. For example, `mylist.append('an
+item')` will add that string to the list `mylist`. Note the use of
+dotted notation for accessing methods of the objects.
+
+A class can also have **fields** which are nothing but variables
+defined for use with respect to that class only. You can use these
+variables/names only when you have an object of that class. Fields are
+also accessed by the dotted notation, for example, `mylist.field`.
+
+Example (save as `using_list.py`):
+
+~~~python
+# This is my shopping list #
+shoplist = ['apple', 'mango', 'carrot', 'banana']
+
+print('I have', len(shoplist), 'items to purchase.')
+
+print('These items are:', end=' ')
+for item in shoplist:
+    print(item, end=' ')
+
+print('\nI also have to buy rice.')
+shoplist.append('rice')
+print('My shopping list is now', shoplist)
+
+print('I will sort my list now')
+shoplist.sort()
+print('Sorted shopping list is', shoplist)
+
+print('The first item I will buy is', shoplist[0])
+olditem = shoplist[0]
+del shoplist[0]
+print('I bought the', olditem)
+print('My shopping list is now', shoplist)
+~~~
+
+Output:
+
+~~~
+$ python3 using_list.py
+I have 4 items to purchase.
+These items are: apple mango carrot banana
+I also have to buy rice.
+My shopping list is now ['apple', 'mango', 'carrot', 'banana', 'rice']
+I will sort my list now
+Sorted shopping list is ['apple', 'banana', 'carrot', 'mango', 'rice']
+The first item I will buy is apple
+I bought the apple
+My shopping list is now ['banana', 'carrot', 'mango', 'rice']
+~~~
+
+How It Works:
+
+The variable `shoplist` is a shopping list for someone who is going to
+the market. In `shoplist`, we only store strings of the names of the
+items to buy but you can add *any kind of object* to a list including
+numbers and even other lists.
+
+We have also used the `for..in` loop to iterate through the items of
+the list. By now, you must have realised that a list is also a
+sequence. The speciality of sequences will be discussed in a
+[later section](#sequence).
+
+Notice the use of the `end` keyword argument to the `print` function
+to indicate that we want to end the output with a space instead of the
+usual line break.
+
+Next, we add an item to the list using the `append` method of the list
+object, as already discussed before. Then, we check that the item has
+been indeed added to the list by printing the contents of the list by
+simply passing the list to the `print` statement which prints it
+neatly.
+
+Then, we sort the list by using the `sort` method of the list. It is
+important to understand that this method affects the list itself and
+does not return a modified list - this is different from the way
+strings work. This is what we mean by saying that lists are *mutable*
+and that strings are *immutable*.
+
+Next, when we finish buying an item in the market, we want to remove
+it from the list. We achieve this by using the `del` statement. Here,
+we mention which item of the list we want to remove and the `del`
+statement removes it from the list for us.  We specify that we want to
+remove the first item from the list and hence we use `del shoplist[0]`
+(remember that Python starts counting from 0).
+
+If you want to know all the methods defined by the list object, see
+`help(list)` for details.
+
+## Tuple ##
+
+Tuples are used to hold together multiple objects. Think of them as
+similar to lists, but without the extensive functionality that the
+list class gives you. One major feature of tuples is that they are
+**immutable** like strings i.e. you cannot modify tuples.
+
+Tuples are defined by specifying items separated by commas within an
+optional pair of parentheses.
+
+Tuples are usually used in cases where a statement or a user-defined
+function can safely assume that the collection of values i.e. the
+tuple of values used will not change.
+
+Example (save as `using_tuple.py`):
+
+~~~python
+zoo = ('python', 'elephant', 'penguin') # remember the parentheses are optional
+print('Number of animals in the zoo is', len(zoo))
+
+new_zoo = 'monkey', 'camel', zoo
+print('Number of cages in the new zoo is', len(new_zoo))
+print('All animals in new zoo are', new_zoo)
+print('Animals brought from old zoo are', new_zoo[2])
+print('Last animal brought from old zoo is', new_zoo[2][2])
+print('Number of animals in the new zoo is', len(new_zoo)-1+len(new_zoo[2]))
+~~~
+
+Output:
+
+~~~
+$ python3 using_tuple.py
+Number of animals in the zoo is 3
+Number of cages in the new zoo is 3
+All animals in new zoo are ('monkey', 'camel', ('python', 'elephant', 'penguin'))
+Animals brought from old zoo are ('python', 'elephant', 'penguin')
+Last animal brought from old zoo is penguin
+Number of animals in the new zoo is 5
+~~~
+
+How It Works:
+
+The variable `zoo` refers to a tuple of items. We see that the `len`
+function can be used to get the length of the tuple. This also
+indicates that a tuple is a [sequence](#sequence) as well.
+
+We are now shifting these animals to a new zoo since the old zoo is
+being closed. Therefore, the `new_zoo` tuple contains some animals
+which are already there along with the animals brought over from the
+old zoo. Back to reality, note that a tuple within a tuple does not
+lose its identity.
+
+We can access the items in the tuple by specifying the item's position
+within a pair of square brackets just like we did for lists. This is
+called the *indexing* operator. We access the third item in `new_zoo`
+by specifying `new_zoo[2]` and we access the third item within the
+third item in the `new_zoo` tuple by specifying `new_zoo[2][2]`. This
+is pretty simple once you've understood the idiom.
+
+Parentheses
+
+:   Although the parentheses are optional, I prefer always having them
+    to make it obvious that it is a tuple, especially because it
+    avoids ambiguity. For example, `print(1,2,3)` and `print( (1,2,3)
+    )` mean two different things - the former prints three numbers
+    whereas the latter prints a tuple (which contains three numbers).
+
+Tuple with 0 or 1 items
+
+:   An empty tuple is constructed by an empty pair of parentheses such
+    as `myempty = ()`. However, a tuple with a single item is not so
+    simple. You have to specify it using a comma following the first
+    (and only) item so that Python can differentiate between a tuple
+    and a pair of parentheses surrounding the object in an expression
+    i.e. you have to specify `singleton = (2 , )` if you mean you want
+    a tuple containing the item `2`.
+
+Note for Perl programmers
+
+:  A list within a list does not lose its identity i.e. lists are not
+   flattened as in Perl. The same applies to a tuple within a tuple,
+   or a tuple within a list, or a list within a tuple, etc. As far as
+   Python is concerned, they are just objects stored using another
+   object, that's all.
+
+## Dictionary ##
+
+A dictionary is like an address-book where you can find the address or
+contact details of a person by knowing only his/her name i.e. we
+associate **keys** (name) with **values** (details). Note that the key
+must be unique just like you cannot find out the correct information
+if you have two persons with the exact same name.
+
+Note that you can use only immutable objects (like strings) for the
+keys of a dictionary but you can use either immutable or mutable
+objects for the values of the dictionary.  This basically translates
+to say that you should use only simple objects for keys.
+
+Pairs of keys and values are specified in a dictionary by using the
+notation `d = {key1 : value1, key2 : value2 }`. Notice that the
+key-value pairs are separated by a colon and the pairs are separated
+themselves by commas and all this is enclosed in a pair of curly
+braces.
+
+Remember that key-value pairs in a dictionary are not ordered in any
+manner. If you want a particular order, then you will have to sort
+them yourself before using it.
+
+The dictionaries that you will be using are instances/objects of the
+`dict` class.
+
+Example (save as `using_dict.py`):
+
+~~~python
+# 'ab' is short for 'a'ddress'b'ook
+
+ab = {  'Swaroop'   : 'swaroop@swaroopch.com',
+        'Larry'     : 'larry@wall.org',
+        'Matsumoto' : 'matz@ruby-lang.org',
+        'Spammer'   : 'spammer@hotmail.com'
+    }
+
+print("Swaroop's address is", ab['Swaroop'])
+
+# Deleting a key-value pair
+del ab['Spammer']
+
+print('\nThere are {0} contacts in the address-book\n'.format(len(ab)))
+
+for name, address in ab.items():
+    print('Contact {0} at {1}'.format(name, address))
+
+# Adding a key-value pair
+ab['Guido'] = 'guido@python.org'
+
+if 'Guido' in ab:
+    print("\nGuido's address is", ab['Guido'])
+~~~
+
+Output:
+
+~~~
+$ python3 using_dict.py
+Swaroop's address is swaroop@swaroopch.com
+
+There are 3 contacts in the address-book
+
+Contact Swaroop at swaroop@swaroopch.com
+Contact Matsumoto at matz@ruby-lang.org
+Contact Larry at larry@wall.org
+
+Guido's address is guido@python.org
+~~~
+
+How It Works:
+
+We create the dictionary `ab` using the notation already discussed. We
+then access key-value pairs by specifying the key using the indexing
+operator as discussed in the context of lists and tuples. Observe the
+simple syntax.
+
+We can delete key-value pairs using our old friend - the `del`
+statement. We simply specify the dictionary and the indexing operator
+for the key to be removed and pass it to the `del` statement. There is
+no need to know the value corresponding to the key for this operation.
+
+Next, we access each key-value pair of the dictionary using the
+`items` method of the dictionary which returns a list of tuples where
+each tuple contains a pair of items - the key followed by the
+value. We retrieve this pair and assign it to the variables `name` and
+`address` correspondingly for each pair using the `for..in` loop and
+then print these values in the for-block.
+
+We can add new key-value pairs by simply using the indexing operator
+to access a key and assign that value, as we have done for Guido in
+the above case.
+
+We can check if a key-value pair exists using the `in` operator.
+
+For the list of methods of the `dict` class, see `help(dict)`.
+
+Keyword Arguments and Dictionaries
+
+:   On a different note, if you have used keyword arguments in your
+    functions, you have already used dictionaries! Just think about
+    it - the key-value pair is specified by you in the parameter list
+    of the function definition and when you access variables within
+    your function, it is just a key access of a dictionary (which is
+    called the *symbol table* in compiler design terminology).
+
+## Sequence ##
+
+Lists, tuples and strings are examples of sequences, but what are
+sequences and what is so special about them?
+
+The major features are **membership tests**, (i.e. the `in` and `not
+in` expressions) and **indexing operations**, which allow us to fetch
+a particular item in the sequence directly.
+
+The three types of sequences mentioned above - lists, tuples and
+strings, also have a **slicing** operation which allows us to retrieve
+a slice of the sequence i.e. a part of the sequence.
+
+Example (save as `seq.py`):
+
+~~~python
+shoplist = ['apple', 'mango', 'carrot', 'banana']
+name = 'swaroop'
+
+# Indexing or 'Subscription' operation #
+print('Item 0 is', shoplist[0])
+print('Item 1 is', shoplist[1])
+print('Item 2 is', shoplist[2])
+print('Item 3 is', shoplist[3])
+print('Item -1 is', shoplist[-1])
+print('Item -2 is', shoplist[-2])
+print('Character 0 is', name[0])
+
+# Slicing on a list #
+print('Item 1 to 3 is', shoplist[1:3])
+print('Item 2 to end is', shoplist[2:])
+print('Item 1 to -1 is', shoplist[1:-1])
+print('Item start to end is', shoplist[:])
+
+# Slicing on a string #
+print('characters 1 to 3 is', name[1:3])
+print('characters 2 to end is', name[2:])
+print('characters 1 to -1 is', name[1:-1])
+print('characters start to end is', name[:])
+~~~
+
+Output:
+
+~~~
+$ python3 seq.py
+Item 0 is apple
+Item 1 is mango
+Item 2 is carrot
+Item 3 is banana
+Item -1 is banana
+Item -2 is carrot
+Character 0 is s
+Item 1 to 3 is ['mango', 'carrot']
+Item 2 to end is ['carrot', 'banana']
+Item 1 to -1 is ['mango', 'carrot']
+Item start to end is ['apple', 'mango', 'carrot', 'banana']
+characters 1 to 3 is wa
+characters 2 to end is aroop
+characters 1 to -1 is waroo
+characters start to end is swaroop
+~~~
+
+How It Works:
+
+First, we see how to use indexes to get individual items of a
+sequence. This is also referred to as the *subscription
+operation*. Whenever you specify a number to a sequence within square
+brackets as shown above, Python will fetch you the item corresponding
+to that position in the sequence. Remember that Python starts counting
+numbers from 0. Hence, `shoplist[0]` fetches the first item and
+`shoplist[3]` fetches the fourth item in the `shoplist`sequence.
+
+The index can also be a negative number, in which case, the position
+is calculated from the end of the sequence. Therefore, `shoplist[-1]`
+refers to the last item in the sequence and `shoplist[-2]` fetches the
+second last item in the sequence.
+
+The slicing operation is used by specifying the name of the sequence
+followed by an optional pair of numbers separated by a colon within
+square brackets. Note that this is very similar to the indexing
+operation you have been using till now. Remember the numbers are
+optional but the colon isn't.
+
+The first number (before the colon) in the slicing operation refers to
+the position from where the slice starts and the second number (after
+the colon) indicates where the slice will stop at. If the first number
+is not specified, Python will start at the beginning of the
+sequence. If the second number is left out, Python will stop at the
+end of the sequence. Note that the slice returned *starts* at the
+start position and will end just before the *end* position i.e. the
+start position is included but the end position is excluded from the
+sequence slice.
+
+Thus, `shoplist[1:3]` returns a slice of the sequence starting at
+position 1, includes position 2 but stops at position 3 and therefore
+a *slice* of two items is returned.  Similarly, `shoplist[:]` returns
+a copy of the whole sequence.
+
+You can also do slicing with negative positions. Negative numbers are
+used for positions from the end of the sequence. For example,
+`shoplist[:-1]` will return a slice of the sequence which excludes the
+last item of the sequence but contains everything else.
+
+You can also provide a third argument for the slice, which is the
+*step* for the slicing (by default, the step size is 1):
+
+~~~
+>>> shoplist = ['apple', 'mango', 'carrot', 'banana']
+>>> shoplist[::1]
+['apple', 'mango', 'carrot', 'banana']
+>>> shoplist[::2]
+['apple', 'carrot']
+>>> shoplist[::3]
+['apple', 'banana']
+>>> shoplist[::-1]
+['banana', 'carrot', 'mango', 'apple']
+~~~
+
+Notice that when the step is 2, we get the items with position 0, 2,
+... When the step size is 3, we get the items with position 0, 3, etc.
+
+Try various combinations of such slice specifications using the Python
+interpreter interactively i.e. the prompt so that you can see the
+results immediately. The great thing about sequences is that you can
+access tuples, lists and strings all in the same way!
+
+## Set ##
+
+Sets are *unordered* collections of simple objects. These are used
+when the existence of an object in a collection is more important than
+the order or how many times it occurs.
+
+Using sets, you can test for membership, whether it is a subset of
+another set, find the intersection between two sets, and so on.
+
+~~~
+>>> bri = set(['brazil', 'russia', 'india'])
+>>> 'india' in bri
+True
+>>> 'usa' in bri
+False
+>>> bric = bri.copy()
+>>> bric.add('china')
+>>> bric.issuperset(bri)
+True
+>>> bri.remove('russia')
+>>> bri & bric # OR bri.intersection(bric)
+{'brazil', 'india'}
+~~~
+
+How It Works:
+
+The example is pretty much self-explanatory because it involves basic
+set theory mathematics taught in school.
+
+## References ##
+
+When you create an object and assign it to a variable, the variable
+only *refers* to the object and does not represent the object itself!
+That is, the variable name points to that part of your computer's
+memory where the object is stored. This is called **binding** the name
+to the object.
+
+Generally, you don't need to be worried about this, but there is a
+subtle effect due to references which you need to be aware of:
+
+Example (save as `reference.py`):
+
+~~~python
+print('Simple Assignment')
+shoplist = ['apple', 'mango', 'carrot', 'banana']
+mylist = shoplist # mylist is just another name pointing to the same object!
+
+del shoplist[0] # I purchased the first item, so I remove it from the list
+
+print('shoplist is', shoplist)
+print('mylist is', mylist)
+# notice that both shoplist and mylist both print the same list without #
+# the 'apple' confirming that they point to the same object #
+
+print('Copy by making a full slice')
+mylist = shoplist[:] # make a copy by doing a full slice
+del mylist[0] # remove first item
+
+print('shoplist is', shoplist)
+print('mylist is', mylist)
+# notice that now the two lists are different #
+~~~
+
+Output:
+
+~~~
+$ python3 reference.py
+Simple Assignment
+shoplist is ['mango', 'carrot', 'banana']
+mylist is ['mango', 'carrot', 'banana']
+Copy by making a full slice
+shoplist is ['mango', 'carrot', 'banana']
+mylist is ['carrot', 'banana']
+~~~
+
+How It Works:
+
+Most of the explanation is available in the comments.
+
+Remember that if you want to make a copy of a list or such kinds of
+sequences or complex objects (not simple *objects* such as integers),
+then you have to use the slicing operation to make a copy. If you just
+assign the variable name to another name, both of them will ''refer''
+to the same object and this could be trouble if you are not careful.
+
+Note for Perl programmers
+
+:   Remember that an assignment statement for lists does **not**
+    create a copy. You have to use slicing operation to make a copy of
+    the sequence.
+
+## More About Strings ##
+
+We have already discussed strings in detail earlier. What more can
+there be to know?  Well, did you know that strings are also objects
+and have methods which do everything from checking part of a string to
+stripping spaces!
+
+The strings that you use in program are all objects of the class
+`str`.  Some useful methods of this class are demonstrated in the next
+example. For a complete list of such methods, see `help(str)`.
+
+Example (save as `str_methods.py`):
+
+~~~python
+name = 'Swaroop' # This is a string object
+
+if name.startswith('Swa'):
+    print('Yes, the string starts with "Swa"')
+
+if 'a' in name:
+    print('Yes, it contains the string "a"')
+
+if name.find('war') != -1:
+    print('Yes, it contains the string "war"')
+
+delimiter = '_*_'
+mylist = ['Brazil', 'Russia', 'India', 'China']
+print(delimiter.join(mylist))
+
+~~~
+
+Output:
+
+~~~
+$ python3 str_methods.py
+Yes, the string starts with "Swa"
+Yes, it contains the string "a"
+Yes, it contains the string "war"
+Brazil_*_Russia_*_India_*_China
+~~~
+
+How It Works:
+
+Here, we see a lot of the string methods in action. The `startswith`
+method is used to find out whether the string starts with the given
+string. The `in` operator is used to check if a given string is a part
+of the string.
+
+The `find` method is used to locate the position of the given
+substring within the string; `find` returns -1 if it is unsuccessful
+in finding the substring. The `str` class also has a neat method to
+`join` the items of a sequence with the string acting as a delimiter
+between each item of the sequence and returns a bigger string
+generated from this.
+
+## Summary ##
+
+We have explored the various built-in data structures of Python in
+detail. These data structures will be essential for writing programs
+of reasonable size.
+
+Now that we have a lot of the basics of Python in place, we will next
+see how to design and write a real-world Python program.

12-problem-solving.md

@@ -0,0 +1,447 @@
+# Problem Solving #
+
+We have explored various parts of the Python language and now we will
+take a look at how all these parts fit together, by designing and
+writing a program which *does* something useful. The idea is to learn
+how to write a Python script on your own.
+
+## The Problem ##
+
+The problem we want to solve is *"I want a program which creates a
+backup of all my important files"*.
+
+Although, this is a simple problem, there is not enough information
+for us to get started with the solution. A little more **analysis** is
+required. For example, how do we specify *which* files are to be
+backed up? *How* are they stored? *Where* are they stored?
+
+After analyzing the problem properly, we **design** our program. We
+make a list of things about how our program should work. In this case,
+I have created the following list on how *I* want it to work. If you
+do the design, you may not come up with the same kind of analysis
+since every person has their own way of doing things, so that is
+perfectly okay.
+
+- The files and directories to be backed up are specified in a list.
+- The backup must be stored in a main backup directory.
+- The files are backed up into a zip file.
+- The name of the zip archive is the current date and time.
+- We use the standard `zip` command available by default in any
+  standard Linux/Unix distribution. Windows users can
+  [install](http://gnuwin32.sourceforge.net/downlinks/zip.php) from
+  the
+  [GnuWin32 project page](http://gnuwin32.sourceforge.net/packages/zip.htm)
+  and add `C:\Program Files\GnuWin32\bin` to your system PATH
+  environment variable, similar to
+  [what we did for recognizing the python command itself](#dos-prompt). Note
+  that you can use any archiving command you want as long as it has a
+  command line
+
+## The Solution ##
+
+As the design of our program is now reasonably stable, we can write
+the code which is an **implementation** of our solution.
+
+Save as `backup_ver1.py`:
+
+~~~python
+import os
+import time
+
+# 1. The files and directories to be backed up are specified in a list. #
+source = ['"C:\\My Documents"', 'C:\\Code']
+# Notice we had to use double quotes inside the string for names with spaces in it. #
+
+# 2. The backup must be stored in a main backup directory #
+target_dir = 'E:\\Backup' # Remember to change this to what you will be using
+
+# 3. The files are backed up into a zip file. #
+# 4. The name of the zip archive is the current date and time #
+target = target_dir + os.sep + time.strftime('%Y%m%d%H%M%S') + '.zip'
+
+# 5. We use the zip command to put the files in a zip archive #
+zip_command = "zip -r {0} {1}".format(target, ' '.join(source))
+
+# Run the backup #
+if os.system(zip_command) == 0:
+    print('Successful backup to', target)
+else:
+    print('Backup FAILED')
+~~~
+
+Output:
+
+~~~
+$ python backup_ver1.py
+Successful backup to E:\Backup\20080702185040.zip
+~~~
+
+Now, we are in the **testing** phase where we test that our program
+works properly. If it doesn't behave as expected, then we have to
+**debug** our program i.e. remove the *bugs* (errors) from the
+program.
+
+If the above program does not work for you, put a `print(zip_command)`
+just before the `os.system` call and run the program. Now copy/paste
+the printed zip_command to the shell prompt and see if it runs
+properly on its own. If this command fails, check the zip command
+manual on what could be wrong. If this command succeeds, then check
+the Python program if it exactly matches the program written above.
+
+How It Works:
+
+You will notice how we have converted our *design* into *code* in a
+step-by-step manner.
+
+We make use of the `os` and `time` modules by first importing
+them. Then, we specify the files and directories to be backed up in
+the `source` list. The target directory is where we store all the
+backup files and this is specified in the `target_dir` variable. The
+name of the zip archive that we are going to create is the current
+date and time which we generate using the `time.strftime()`
+function. It will also have the `.zip` extension and will be stored in
+the `target_dir` directory.
+
+Notice the use of the `os.sep` variable - this gives the directory
+separator according to your operating system i.e. it will be `'/'` in
+Linux and Unix, it will be `'\\'` in Windows and `':'` in Mac
+OS. Using `os.sep` instead of these characters directly will make our
+program portable and work across all of these systems.
+
+The `time.strftime()` function takes a specification such as the one
+we have used in the above program. The `%Y` specification will be
+replaced by the year with the century. The `%m` specification will be
+replaced by the month as a decimal number between `01` and `12` and so
+on. The complete list of such specifications can be found in the
+[Python Reference Manual](http://docs.python.org/3/library/time.html#time.strftime).
+
+We create the name of the target zip file using the addition operator
+which *concatenates* the strings i.e. it joins the two strings
+together and returns a new one. Then, we create a string `zip_command`
+which contains the command that we are going to execute. You can check
+if this command works by running it in the shell (Linux terminal or
+DOS prompt).
+
+The `zip` command that we are using has some options and parameters
+passed. The `-q` option is used to indicate that the zip command
+should work **q**uietly.  The `-r` option specifies that the zip
+command should work **r**ecursively for directories i.e. it should
+include all the subdirectories and files. The two options are combined
+and specified in a shortcut as `-qr`. The options are followed by the
+name of the zip archive to create followed by the list of files and
+directories to backup. We convert the `source` list into a string
+using the `join` method of strings which we have already seen how to
+use.
+
+Then, we finally *run* the command using the `os.system` function
+which runs the command as if it was run from the *system* i.e. in the
+shell - it returns `0` if the command was successfully, else it
+returns an error number.
+
+Depending on the outcome of the command, we print the appropriate
+message that the backup has failed or succeeded.
+
+That's it, we have created a script to take a backup of our important
+files!
+
+Note to Windows Users
+
+:   Instead of double backslash escape sequences, you can also use raw
+    strings. For example, use `'C:\\Documents'` or
+    `r'C:\Documents'`. However, do **not** use `'C:\Documents'` since
+    you end up using an unknown escape sequence `\D`.
+
+Now that we have a working backup script, we can use it whenever we
+want to take a backup of the files. Linux/Unix users are advised to
+use the [executable method](#executable-python-programs) as discussed
+earlier so that they can run the backup script anytime anywhere. This
+is called the **operation** phase or the **deployment** phase of the
+software.
+
+The above program works properly, but (usually) first programs do not
+work exactly as you expect. For example, there might be problems if
+you have not designed the program properly or if you have made a
+mistake when typing the code, etc. Appropriately, you will have to go
+back to the design phase or you will have to debug your program.
+
+## Second Version ##
+
+The first version of our script works. However, we can make some
+refinements to it so that it can work better on a daily basis. This is
+called the **maintenance** phase of the software.
+
+One of the refinements I felt was useful is a better file-naming
+mechanism - using the *time* as the name of the file within a
+directory with the current *date* as a directory within the main
+backup directory. The first advantage is that your backups are stored
+in a hierarchical manner and therefore it is much easier to
+manage. The second advantage is that the filenames are much
+shorter. The third advantage is that separate directories will help
+you check if you have made a backup for each day since the directory
+would be created only if you have made a backup for that day.
+
+Save as `backup_ver2.py`:
+
+~~~python
+import os
+import time
+
+# 1. The files and directories to be backed up are specified in a list. #
+source = ['"C:\\My Documents"', 'C:\\Code']
+# Notice we had to use double quotes inside the string for names with spaces in it. #
+
+# 2. The backup must be stored in a main backup directory #
+target_dir = 'E:\\Backup' # Remember to change this to what you will be using
+
+# 3. The files are backed up into a zip file. #
+# 4. The current day is the name of the subdirectory in the main directory #
+today = target_dir + os.sep + time.strftime('%Y%m%d')
+# The current time is the name of the zip archive #
+now = time.strftime('%H%M%S')
+
+# Create the subdirectory if it isn't already there #
+if not os.path.exists(today):
+    os.mkdir(today) # make directory
+    print('Successfully created directory', today)
+
+# The name of the zip file #
+target = today + os.sep + now + '.zip'
+
+# 5. We use the zip command to put the files in a zip archive #
+zip_command = "zip -r {0} {1}".format(target, ' '.join(source))
+
+# Run the backup #
+if os.system(zip_command) == 0:
+    print('Successful backup to', target)
+else:
+    print('Backup FAILED')
+~~~
+
+Output:
+
+~~~
+$ python3 backup_ver2.py
+Successfully created directory E:\Backup\20080702
+Successful backup to E:\Backup\20080702\202311.zip
+
+$ python3 backup_ver2.py
+Successful backup to E:\Backup\20080702\202325.zip
+~~~
+
+How It Works:
+
+Most of the program remains the same. The changes are that we check if
+there is a directory with the current day as its name inside the main
+backup directory using the `os.path.exists` function. If it doesn't
+exist, we create it using the `os.mkdir` function.
+
+## Third Version ##
+
+The second version works fine when I do many backups, but when there
+are lots of backups, I am finding it hard to differentiate what the
+backups were for! For example, I might have made some major changes to
+a program or presentation, then I want to associate what those changes
+are with the name of the zip archive. This can be easily achieved by
+attaching a user-supplied comment to the name of the zip archive.
+
+Note
+
+:   The following program does not work, so do not be alarmed, please
+    follow along because there's a lesson in here.
+
+Save as `backup_ver3.py`:
+
+~~~python
+import os
+import time
+
+# 1. The files and directories to be backed up are specified in a list. #
+source = ['"C:\\My Documents"', 'C:\\Code']
+# Notice we had to use double quotes inside the string for names with spaces in it. #
+
+# 2. The backup must be stored in a main backup directory #
+target_dir = 'E:\\Backup' # Remember to change this to what you will be using
+
+# 3. The files are backed up into a zip file. #
+# 4. The current day is the name of the subdirectory in the main directory #
+today = target_dir + os.sep + time.strftime('%Y%m%d')
+# The current time is the name of the zip archive #
+now = time.strftime('%H%M%S')
+
+# Take a comment from the user to create the name of the zip file #
+comment = input('Enter a comment --> ')
+if len(comment) == 0: # check if a comment was entered
+    target = today + os.sep + now + '.zip'
+else:
+    target = today + os.sep + now + '_' +
+        comment.replace(' ', '_') + '.zip'
+
+# Create the subdirectory if it isn't already there #
+if not os.path.exists(today):
+    os.mkdir(today) # make directory
+    print('Successfully created directory', today)
+
+# 5. We use the zip command to put the files in a zip archive #
+zip_command = "zip -r {0} {1}".format(target, ' '.join(source))
+
+# Run the backup #
+if os.system(zip_command) == 0:
+    print('Successful backup to', target)
+else:
+    print('Backup FAILED')
+~~~
+
+Output:
+
+~~~
+$ python3 backup_ver3.py
+  File "backup_ver3.py", line 25
+    target = today + os.sep + now + '_' +
+                                        ^
+SyntaxError: invalid syntax
+~~~
+
+How This (does not) Work:
+
+**This program does not work!** Python says there is a syntax error
+which means that the script does not satisfy the structure that
+Python expects to see. When we observe the error given by Python, it
+also tells us the place where it detected the error as well. So we
+start *debugging* our program from that line.
+
+On careful observation, we see that the single logical line has been
+split into two physical lines but we have not specified that these two
+physical lines belong together. Basically, Python has found the
+addition operator (`+`) without any operand in that logical line and
+hence it doesn't know how to continue. Remember that we can specify
+that the logical line continues in the next physical line by the use
+of a backslash at the end of the physical line. So, we make this
+correction to our program. This correction of the program when we find
+errors is called **bug fixing**.
+
+## Fourth Version ##
+
+Save as `backup_ver4.py`:
+
+~~~python
+import os
+import time
+
+# 1. The files and directories to be backed up are specified in a list. #
+source = ['"C:\\My Documents"', 'C:\\Code']
+# Notice we had to use double quotes inside the string for names with spaces in it. #
+
+# 2. The backup must be stored in a main backup directory #
+target_dir = 'E:\\Backup' # Remember to change this to what you will be using
+
+# 3. The files are backed up into a zip file. #
+# 4. The current day is the name of the subdirectory in the main directory #
+today = target_dir + os.sep + time.strftime('%Y%m%d')
+# The current time is the name of the zip archive #
+now = time.strftime('%H%M%S')
+
+# Take a comment from the user to create the name of the zip file #
+comment = input('Enter a comment --> ')
+if len(comment) == 0: # check if a comment was entered
+    target = today + os.sep + now + '.zip'
+else:
+    target = today + os.sep + now + '_' + \
+        comment.replace(' ', '_') + '.zip'
+
+# Create the subdirectory if it isn't already there #
+if not os.path.exists(today):
+    os.mkdir(today) # make directory
+    print('Successfully created directory', today)
+
+# 5. We use the zip command to put the files in a zip archive #
+zip_command = "zip -r {0} {1}".format(target, ' '.join(source))
+
+# Run the backup #
+if os.system(zip_command) == 0:
+    print('Successful backup to', target)
+else:
+    print('Backup FAILED')
+~~~
+
+Output:
+
+~~~
+$ python3 backup_ver4.py
+Enter a comment --> added new examples
+Successful backup to E:\Backup\20080702\202836_added_new_examples.zip
+
+$ python3 backup_ver4.py
+Enter a comment -->
+Successful backup to E:\Backup\20080702\202839.zip
+~~~
+
+How It Works:
+
+This program now works! Let us go through the actual enhancements that
+we had made in version 3. We take in the user's comments using the
+`input` function and then check if the user actually entered something
+by finding out the length of the input using the `len` function. If
+the user has just pressed `enter` without entering anything (maybe it
+was just a routine backup or no special changes were made), then we
+proceed as we have done before.
+
+However, if a comment was supplied, then this is attached to the name
+of the zip archive just before the `.zip` extension.  Notice that we
+are replacing spaces in the comment with underscores - this is because
+managing filenames without spaces is much easier.
+
+## More Refinements ##
+
+The fourth version is a satisfactorily working script for most users,
+but there is always room for improvement. For example, you can include
+a *verbosity* level for the program where you can specify a `-v`
+option to make your program become more talkative or a `-q` to make it
+*quiet*.
+
+Another possible enhancement would be to allow extra files and
+directories to be passed to the script at the command line. We can get
+these names from the `sys.argv` list and we can add them to our
+`source` list using the `extend`method provided by the `list` class.
+
+The most important refinement would be to not use the `os.system` way
+of creating archives and instead using the `zipfile` or `tarfile`
+built-in module to create these archives. They are part of the
+standard library and available already for you to use without external
+dependencies on the zip program to be available on your computer.
+
+However, I have been using the `os.system` way of creating a backup in
+the above examples purely for pedagogical purposes, so that the
+example is simple enough to be understood by everybody but real enough
+to be useful.
+
+Can you try writing the fifth version that uses the
+[zipfile](http://docs.python.org/3/library/zipfile.html) module
+instead of the `os.system` call?
+
+## The Software Development Process ##
+
+We have now gone through the various **phases** in the process of
+writing a software. These phases can be summarised as follows:
+
+1. What (Analysis)
+2. How (Design)
+3. Do It (Implementation)
+4. Test (Testing and Debugging)
+5. Use (Operation or Deployment)
+6. Maintain (Refinement)
+
+A recommended way of writing programs is the procedure we have
+followed in creating the backup script: Do the analysis and
+design. Start implementing with a simple version. Test and debug
+it. Use it to ensure that it works as expected. Now, add any features
+that you want and continue to repeat the Do It-Test-Use cycle as many
+times as required. Remember, **Software is grown, not built**.
+
+## Summary ##
+
+We have seen how to create our own Python programs/scripts and the
+various stages involved in writing such programs. You may find it
+useful to create your own program just like we did in this chapter so
+that you become comfortable with Python as well as problem-solving.
+
+Next, we will discuss object-oriented programming.

13-oop.md

@@ -0,0 +1,514 @@
+# Object Oriented Programming #
+
+In all the programs we wrote till now, we have designed our program
+around functions i.e. blocks of statements which manipulate data. This
+is called the *procedure-oriented* way of programming. There is
+another way of organizing your program which is to combine data and
+functionality and wrap it inside something called an object. This is
+called the *object oriented* programming paradigm. Most of the time
+you can use procedural programming, but when writing large programs or
+have a problem that is better suited to this method, you can use
+object oriented programming techniques.
+
+Classes and objects are the two main aspects of object oriented
+programming. A **class** creates a new *type* where **objects** are
+*instances* of the class. An analogy is that you can have variables of
+type `int` which translates to saying that variables that store
+integers are variables which are instances (objects) of the `int`
+class.
+
+Note for Static Language Programmers
+
+:   Note that even integers are treated as objects (of the `int`
+    class). This is unlike C++ and Java (before version 1.5) where
+    integers are primitive native types.
+
+    See `help(int)` for more details on the class.
+
+    C# and Java 1.5 programmers will find this similar to the *boxing
+    and unboxing* concept.
+
+Objects can store data using ordinary variables that *belong* to the
+object. Variables that belong to an object or class are referred to as
+**fields**. Objects can also have functionality by using functions
+that *belong* to a class. Such functions are called **methods** of the
+class. This terminology is important because it helps us to
+differentiate between functions and variables which are independent
+and those which belong to a class or object. Collectively, the fields
+and methods can be referred to as the **attributes** of that class.
+
+Fields are of two types - they can belong to each instance/object of
+the class or they can belong to the class itself. They are called
+**instance variables** and **class variables** respectively.
+
+A class is created using the `class` keyword. The fields and methods
+of the class are listed in an indented block.
+
+## The self ##
+
+Class methods have only one specific difference from ordinary
+functions - they must have an extra first name that has to be added to
+the beginning of the parameter list, but you **do not** give a value
+for this parameter when you call the method, Python will provide
+it. This particular variable refers to the object *itself*, and by
+convention, it is given the name `self`.
+
+Although, you can give any name for this parameter, it is *strongly
+recommended* that you use the name `self` - any other name is
+definitely frowned upon. There are many advantages to using a standard
+name - any reader of your program will immediately recognize it and
+even specialized IDEs (Integrated Development Environments) can help
+you if you use `self`.
+
+Note for C++/Java/C# Programmers
+
+:   The `self` in Python is equivalent to the `this` pointer in C++
+    and the `this` reference in Java and C#.
+
+You must be wondering how Python gives the value for `self` and why
+you don't need to give a value for it. An example will make this
+clear. Say you have a class called `MyClass` and an instance of this
+class called `myobject`. When you call a method of this object as `
+myobject.method(arg1, arg2)`, this is automatically converted by
+Python into `MyClass.method(myobject, arg1, arg2)` - this is all the
+special `self` is about.
+
+This also means that if you have a method which takes no arguments,
+then you still have to have one argument - the `self`.
+
+## Classes ##
+
+The simplest class possible is shown in the following example (save as
+`simplestclass.py`).
+
+~~~python
+class Person:
+    pass # An empty block
+
+p = Person()
+print(p)
+~~~
+
+Output:
+
+~~~
+$ python3 simplestclass.py
+<__main__.Person object at 0x019F85F0>
+~~~
+
+How It Works:
+
+We create a new class using the `class` statement and the name of the
+class. This is followed by an indented block of statements which form
+the body of the class. In this case, we have an empty block which is
+indicated using the `pass` statement.
+
+Next, we create an object/instance of this class using the name of the
+class followed by a pair of parentheses. (We will learn
+[more about instantiation](#the-init-method) in the next section). For
+our verification, we confirm the type of the variable by simply
+printing it. It tells us that we have an instance of the `Person`
+class in the `__main__` module.
+
+Notice that the address of the computer memory where your object is
+stored is also printed. The address will have a different value on
+your computer since Python can store the object wherever it finds
+space.
+
+## Object Methods ##
+
+We have already discussed that classes/objects can have methods just
+like functions except that we have an extra `self` variable. We will
+now see an example (save as `method.py`).
+
+~~~python
+class Person:
+    def sayHi(self):
+        print('Hello, how are you?')
+
+p = Person()
+p.sayHi()
+
+# This short example can also be written as Person().sayHi() #
+~~~
+
+Output:
+
+~~~
+$ python3 method.py
+Hello, how are you?
+~~~
+
+How It Works:
+
+Here we see the `self` in action. Notice that the `sayHi` method takes
+no parameters but still has the `self` in the function definition.
+
+## The __init__ method ##
+
+There are many method names which have special significance in Python
+classes. We will see the significance of the `__init__` method now.
+
+The `__init__` method is run as soon as an object of a class is
+instantiated. The method is useful to do any *initialization* you want
+to do with your object. Notice the double underscores both at the
+beginning and at the end of the name.
+
+Example (save as `class_init.py`):
+
+~~~python
+class Person:
+    def __init__(self, name):
+        self.name = name
+    def sayHi(self):
+        print('Hello, my name is', self.name)
+
+p = Person('Swaroop')
+p.sayHi()
+
+# This short example can also be written as Person('Swaroop').sayHi() #
+~~~
+
+Output:
+
+~~~
+$ python3 class_init.py
+Hello, my name is Swaroop
+~~~
+
+How It Works:
+
+Here, we define the `__init__` method as taking a parameter `name`
+(along with the usual `self`).  Here, we just create a new field also
+called `name`. Notice these are two different variables even though
+they are both called 'name'. There is no problem because the dotted
+notation `self.name` means that there is something called "name" that
+is part of the object called "self" and the other `name` is a local
+variable. Since we explicitly indicate which name we are referring to,
+there is no confusion.
+
+Most importantly, notice that we do not explicitly call the `__init__`
+method but pass the arguments in the parentheses following the class
+name when creating a new instance of the class. This is the special
+significance of this method.
+
+Now, we are able to use the `self.name` field in our methods which is
+demonstrated in the `sayHi` method.
+
+## Class And Object Variables ##
+
+We have already discussed the functionality part of classes and
+objects (i.e. methods), now let us learn about the data part. The data
+part, i.e. fields, are nothing but ordinary variables that are *bound*
+to the **namespaces** of the classes and objects. This means that
+these names are valid within the context of these classes and objects
+only. That's why they are called *name spaces*.
+
+There are two types of *fields* - class variables and object variables
+which are classified depending on whether the class or the object
+*owns* the variables respectively.
+
+*Class variables* are shared - they can be accessed by all instances
+of that class. There is only one copy of the class variable and when
+any one object makes a change to a class variable, that change will
+be seen by all the other instances.
+
+*Object variables* are owned by each individual object/instance of the
+class. In this case, each object has its own copy of the field
+i.e. they are not shared and are not related in any way to the field
+by the same name in a different instance. An example will make this
+easy to understand (save as `objvar.py`):
+
+~~~python
+class Robot:
+    '''Represents a robot, with a name.'''
+
+    # A class variable, counting the number of robots
+    population = 0
+ 
+    def __init__(self, name):
+        '''Initializes the data.'''
+        self.name = name
+        print('(Initializing {0})'.format(self.name))
+ 
+        # When this person is created, the robot
+        # adds to the population
+        Robot.population += 1
+ 
+    def __del__(self):
+        '''I am dying.'''
+        print('{0} is being destroyed!'.format(self.name))
+ 
+        Robot.population -= 1
+ 
+        if Robot.population == 0:
+            print('{0} was the last one.'.format(self.name))
+        else:
+            print('There are still {0:d} robots working.'.format(Robot.population))
+ 
+    def sayHi(self):
+        '''Greeting by the robot.
+ 
+        Yeah, they can do that.'''
+        print('Greetings, my masters call me {0}.'.format(self.name))
+
+    def howMany():
+        '''Prints the current population.'''
+        print('We have {0:d} robots.'.format(Robot.population))
+    howMany = staticmethod(howMany)
+ 
+droid1 = Robot('R2-D2')
+droid1.sayHi()
+Robot.howMany()
+ 
+droid2 = Robot('C-3PO')
+droid2.sayHi()
+Robot.howMany()
+ 
+print("\nRobots can do some work here.\n")
+
+print("Robots have finished their work. So let's destroy them.")
+del droid1
+del droid2
+
+Robot.howMany()
+~~~
+
+Output:
+
+~~~
+$ python3 objvar.py
+(Initializing R2-D2)
+Greetings, my masters call me R2-D2.
+We have 1 robots.
+(Initializing C-3PO)
+Greetings, my masters call me C-3PO.
+We have 2 robots.
+
+Robots can do some work here.
+
+Robots have finished their work. So let's destroy them.
+R2-D2 is being destroyed!
+There are still 1 robots working.
+C-3PO is being destroyed!
+C-3PO was the last one.
+We have 0 robots.
+~~~
+
+How It Works:
+
+This is a long example but helps demonstrate the nature of class and
+object variables. Here, `population` belongs to the`Robot` class and
+hence is a class variable. The `name` variable belongs to the object
+(it is assigned using `self`) and hence is an object variable.
+
+Thus, we refer to the `population` class variable as
+`Robot.population` and not as `self.population`. We refer to the
+object variable `name` using `self.name` notation in the methods of
+that object. Remember this simple difference between class and object
+variables. Also note that an object variable with the same name as a
+class variable will hide the class variable!
+
+The `howMany` is actually a method that belongs to the class and not
+to the object. This means we can define it as either a `classmethod`
+or a `staticmethod` depending on whether we need to know which class
+we are part of. Since we don't need such information, we will go for
+`staticmethod` .
+
+We could have also achieved the same using
+[decorators](http://www.ibm.com/developerworks/linux/library/l-cpdecor.html):
+
+~~~python
+@staticmethod
+def howMany():
+    '''Prints the current population.'''
+    print('We have {0:d} robots.'.format(Robot.population))
+~~~
+
+Decorators can be imagined to be a shortcut to calling an explicit
+statement, as we have seen in this example.
+
+Observe that the `__init__` method is used to initialize the `Robot`
+instance with a name. In this method, we increase the `population`
+count by 1 since we have one more robot being added. Also observe that
+the values of `self.name` is specific to each object which indicates
+the nature of object variables.
+
+Remember, that you must refer to the variables and methods of the same
+object using the `self` **only**. This is called an *attribute
+reference*.
+
+In this program, we also see the use of **docstrings** for classes as
+well as methods. We can access the class docstring at runtime using
+`Robot.__doc__` and the method docstring as `Robot.sayHi.__doc__`
+
+Just like the `__init__` method, there is another special method
+`__del__` which is called when an object is going to die i.e. it is no
+longer being used and is being returned to the computer system for
+reusing that piece of memory. In this method, we simply decrease the
+`Robot.population` count by 1.
+
+The `__del__` method is run when the object is no longer in use and
+there is no guarantee *when* that method will be run. If you want to
+explicitly see it in action, we have to use the `del` statement which
+is what we have done here.
+
+All class members are public. One exception: If you use data members
+with names using the *double underscore prefix* such as
+`__privatevar`, Python uses name-mangling to effectively make it a
+private variable.
+
+Thus, the convention followed is that any variable that is to be used
+only within the class or object should begin with an underscore and
+all other names are public and can be used by other
+classes/objects. Remember that this is only a convention and is not
+enforced by Python (except for the double underscore prefix).
+
+Note for C++/Java/C# Programmers
+
+:   All class members (including the data members) are *public* and
+    all the methods are *virtual* in Python.
+
+## Inheritance ##
+
+One of the major benefits of object oriented programming is **reuse**
+of code and one of the ways this is achieved is through the
+*inheritance* mechanism. Inheritance can be best imagined as
+implementing a *type and subtype* relationship between classes.
+
+Suppose you want to write a program which has to keep track of the
+teachers and students in a college. They have some common
+characteristics such as name, age and address. They also have specific
+characteristics such as salary, courses and leaves for teachers and,
+marks and fees for students.
+
+You can create two independent classes for each type and process them
+but adding a new common characteristic would mean adding to both of
+these independent classes. This quickly becomes unwieldy.
+
+A better way would be to create a common class called `SchoolMember`
+and then have the teacher and student classes *inherit* from this
+class i.e. they will become sub-types of this type (class) and then we
+can add specific characteristics to these sub-types.
+
+There are many advantages to this approach. If we add/change any
+functionality in `SchoolMember`, this is automatically reflected in
+the subtypes as well. For example, you can add a new ID card field for
+both teachers and students by simply adding it to the SchoolMember
+class. However, changes in the subtypes do not affect other
+subtypes. Another advantage is that if you can refer to a teacher or
+student object as a `SchoolMember` object which could be useful in
+some situations such as counting of the number of school members. This
+is called **polymorphism** where a sub-type can be substituted in any
+situation where a parent type is expected i.e. the object can be
+treated as an instance of the parent class.
+
+Also observe that we *reuse* the code of the parent class and we do
+not need to repeat it in the different classes as we would have had to
+in case we had used independent classes.
+
+The `SchoolMember` class in this situation is known as the *base
+class* or the *superclass*. The `Teacher` and `Student` classes are
+called the *derived classes* or *subclasses*.
+
+We will now see this example as a program (save as `inherit.py`):
+
+~~~python
+class SchoolMember:
+    '''Represents any school member.'''
+    def __init__(self, name, age):
+        self.name = name
+        self.age = age
+        print('(Initialized SchoolMember: {0})'.format(self.name))
+    
+    def tell(self):
+        '''Tell my details.'''
+        print('Name:"{0}" Age:"{1}"'.format(self.name, self.age), end=" ")
+
+class Teacher(SchoolMember):
+    '''Represents a teacher.'''
+    def __init__(self, name, age, salary):
+        SchoolMember.__init__(self, name, age)
+        self.salary = salary
+        print('(Initialized Teacher: {0})'.format(self.name))
+
+    def tell(self):
+        SchoolMember.tell(self)
+        print('Salary: "{0:d}"'.format(self.salary))
+
+class Student(SchoolMember):
+    '''Represents a student.'''
+    def __init__(self, name, age, marks):
+        SchoolMember.__init__(self, name, age)
+        self.marks = marks
+        print('(Initialized Student: {0})'.format(self.name))
+    
+    def tell(self):
+        SchoolMember.tell(self)
+        print('Marks: "{0:d}"'.format(self.marks))
+
+t = Teacher('Mrs. Shrividya', 40, 30000)
+s = Student('Swaroop', 25, 75)
+
+print() # prints a blank line
+
+members = [t, s]
+for member in members:
+    member.tell() # works for both Teachers and Students
+~~~
+
+Output:
+
+~~~
+$ python3 inherit.py
+(Initialized SchoolMember: Mrs. Shrividya)
+(Initialized Teacher: Mrs. Shrividya)
+(Initialized SchoolMember: Swaroop)
+(Initialized Student: Swaroop)
+
+Name:"Mrs. Shrividya" Age:"40" Salary: "30000"
+Name:"Swaroop" Age:"25" Marks: "75"
+~~~
+
+How It Works:
+
+To use inheritance, we specify the base class names in a tuple
+following the class name in the class definition. Next, we observe
+that the `__init__` method of the base class is explicitly called
+using the `self` variable so that we can initialize the base class
+part of the object. This is very important to remember - Python does
+not automatically call the constructor of the base class, you have to
+explicitly call it yourself.
+
+We also observe that we can call methods of the base class by
+prefixing the class name to the method call and then pass in the
+`self` variable along with any arguments.
+
+Notice that we can treat instances of `Teacher` or `Student` as just
+instances of the `SchoolMember` when we use the `tell` method of the
+`SchoolMember` class.
+
+Also, observe that the `tell` method of the subtype is called and not
+the `tell` method of the `SchoolMember` class. One way to understand
+this is that Python *always* starts looking for methods in the actual
+type, which in this case it does. If it could not find the method, it
+starts looking at the methods belonging to its base classes one by one
+in the order they are specified in the tuple in the class definition.
+
+A note on terminology - if more than one class is listed in the
+inheritance tuple, then it is called *multiple inheritance*.
+
+The `end` parameter is used in the `tell()` method to change a new
+line to be started at the end of the `print()` call to printing
+spaces.
+
+## Summary ##
+
+We have now explored the various aspects of classes and objects as
+well as the various terminologies associated with it. We have also
+seen the benefits and pitfalls of object-oriented programming. Python
+is highly object-oriented and understanding these concepts carefully
+will help you a lot in the long run.
+
+Next, we will learn how to deal with input/output and how to access
+files in Python.

14-io.md

@@ -0,0 +1,209 @@
+# Input Output #
+
+There will be situations where your program has to interact with the
+user. For example, you would want to take input from the user and then
+print some results back. We can achieve this using the `input()` and
+`print()` functions respectively.
+
+For output, we can also use the various methods of the `str` (string)
+class. For example, you can use the `rjust` method to get a string
+which is right justified to a specified width. See `help(str)` for
+more details.
+
+Another common type of input/output is dealing with files. The ability
+to create, read and write files is essential to many programs and we
+will explore this aspect in this chapter.
+
+## Input from user ##
+
+Save this program as `user_input.py`:
+
+~~~python
+def reverse(text):
+    return text[::-1]
+
+def is_palindrome(text):
+    return text == reverse(text)
+
+something = input('Enter text: ')
+if (is_palindrome(something)):
+    print("Yes, it is a palindrome")
+else:
+    print("No, it is not a palindrome")
+~~~
+
+Output:
+
+~~~
+$ python3 user_input.py
+Enter text: sir
+No, it is not a palindrome
+
+$ python3 user_input.py
+Enter text: madam
+Yes, it is a palindrome
+
+$ python3 user_input.py
+Enter text: racecar
+Yes, it is a palindrome
+~~~
+
+How It Works:
+
+We use the slicing feature to reverse the text. We've already seen how
+we can make [slices from sequences](#sequence) using the `seq[a:b]`
+code starting from position `a` to position `b`. We can also provide a
+third argument that determines the *step* by which the slicing is
+done. The default step is `1` because of which it returns a continuous
+part of the text. Giving a negative step, i.e., `-1` will return the
+text in reverse.
+
+The `input()` function takes a string as argument and displays it to
+the user. Then it waits for the user to type something and press the
+return key. Once the user has entered and pressed the return key, the
+`input()` function will then return that text the user has entered.
+
+We take that text and reverse it. If the original text and reversed
+text are equal, then the text is a
+[palindrome](http://en.wiktionary.org/wiki/palindrome).
+
+Homework exercise
+
+:   Checking whether a text is a palindrome should also ignore
+    punctuation, spaces and case. For example, "Rise to vote, sir." is
+    also a palindrome but our current program doesn't say it is. Can
+    you improve the above program to recognize this palindrome?
+
+    *Hint (Don't read) below*
+
+    Use a tuple (you can find a list of *all* punctuation marks here
+    [punctuation](http://grammar.ccc.commnet.edu/grammar/marks/marks.htm))
+    to hold all the forbidden characters, then use the membership test
+    to determine whether a character should be removed or not,
+    i.e. forbidden = ('!', '?', '.', ...).
+
+## Files ##
+
+You can open and use files for reading or writing by creating an
+object of the `file` class and using its `read`, `readline` or `write`
+methods appropriately to read from or write to the file. The ability
+to read or write to the file depends on the mode you have specified
+for the file opening. Then finally, when you are finished with the
+file, you call the `close` method to tell Python that we are done
+using the file.
+
+Example (save as `using_file.py`):
+
+~~~python
+poem = '''\
+Programming is fun
+When the work is done
+if you wanna make your work also fun:
+    use Python!
+'''
+ 
+f = open('poem.txt', 'w') # open for 'w'riting
+f.write(poem) # write text to file
+f.close() # close the file
+ 
+f = open('poem.txt') # if no mode is specified, 'r'ead mode is assumed by default
+while True:
+    line = f.readline()
+    if len(line) == 0: # Zero length indicates EOF
+        break
+    print(line, end='')
+f.close() # close the file
+~~~
+
+Output:
+
+~~~
+$ python3 using_file.py
+Programming is fun
+When the work is done
+if you wanna make your work also fun:
+        use Python!
+~~~
+
+How It Works:
+
+First, open a file by using the built-in `open` function and
+specifying the name of the file and the mode in which we want to open
+the file. The mode can be a read mode (`'r'`), write mode (`'w'`) or
+append mode (`'a'`). We can also specify whether we are reading,
+writing, or appending in text mode (`'t'`) or binary mode
+(`'b'`). There are actually many more modes available and `help(open)`
+will give you more details about them. By default, `open()` considers
+the file to be a 't'ext file and opens it in 'r'ead mode.
+
+In our example, we first open the file in write text mode and use the
+`write` method of the file object to write to the file and then we
+finally `close` the file.
+
+Next, we open the same file again for reading. We don't need to
+specify a mode because 'read text file' is the default mode. We read
+in each line of the file using the `readline` method in a loop. This
+method returns a complete line including the newline character at the
+end of the line. When an *empty* string is returned, it means that we
+have reached the end of the file and we 'break' out of the loop.
+
+By default, the `print()` function prints the text as well as an
+automatic newline to the screen. We are suppressing the newline by
+specifying `end=''` because the line that is read from the file
+already ends with a newline character. Then, we finally `close`the
+file.
+
+Now, check the contents of the `poem.txt` file to confirm that the
+program has indeed written to and read from that file.
+
+## Pickle ##
+
+Python provides a standard module called `pickle` using which you can
+store **any** plain Python object in a file and then get it back
+later. This is called storing the object *persistently*.
+
+Example (save as `pickling.py`):
+
+~~~python
+import pickle
+ 
+# the name of the file where we will store the object #
+shoplistfile = 'shoplist.data'
+# the list of things to buy #
+shoplist = ['apple', 'mango', 'carrot']
+ 
+# Write to the file #
+f = open(shoplistfile, 'wb')
+pickle.dump(shoplist, f) # dump the object to a file
+f.close()
+ 
+del shoplist # destroy the shoplist variable
+ 
+# Read back from the storage #
+f = open(shoplistfile, 'rb')
+storedlist = pickle.load(f) # load the object from the file
+print(storedlist)
+~~~
+
+Output:
+
+~~~
+$ python3 pickling.py
+['apple', 'mango', 'carrot']
+~~~
+
+How It Works:
+
+To store an object in a file, we have to first `open` the file in
+'w'rite 'b'inary mode and then call the `dump` function of the`pickle`
+module. This process is called *pickling*.
+
+Next, we retrieve the object using the `load` function of the `pickle`
+module which returns the object. This process is called *unpickling*.
+
+## Summary ##
+
+We have discussed various types of input/output and also file handling
+and using the pickle module.
+
+Next, we will explore the concept of exceptions.

15-exceptions.pd

@@ -1,12 +1,19 @@
-# Exceptions
+# Exceptions #
 
-Exceptions occur when certain *exceptional* situations occur in your program.  For example, what if you are going to read a file and the file does not exist? Or what if you accidentally deleted it when the program was running? Such situations are handled using **exceptions**.
+Exceptions occur when certain *exceptional* situations occur in your
+program.  For example, what if you are going to read a file and the
+file does not exist? Or what if you accidentally deleted it when the
+program was running? Such situations are handled using **exceptions**.
 
-Similarly, what if your program had some invalid statements? This is handled by Python which **raises** its hands and tells you there is an **error**.
+Similarly, what if your program had some invalid statements? This is
+handled by Python which **raises** its hands and tells you there is an
+**error**.
 
-## Errors 
+## Errors ##
 
-Consider a simple `print` function call. What if we misspelt `print` as `Print`? Note the capitalization. In this case, Python *raises* a syntax error.
+Consider a simple `print` function call. What if we misspelt `print`
+as `Print`? Note the capitalization. In this case, Python *raises* a
+syntax error.
 
 ~~~
 >>> Print('Hello World')
@@ -18,11 +25,14 @@ NameError: name 'Print' is not defined
 Hello World
 ~~~
 
-Observe that a `NameError` is raised and also the location where the error was detected is printed. This is what an *error handler* for this error does.
+Observe that a `NameError` is raised and also the location where the
+error was detected is printed. This is what an *error handler* for
+this error does.
 
-## Exceptions 
+## Exceptions ##
 
-We will **try** to read input from the user. Press `ctrl-d` and see what happens.
+We will **try** to read input from the user. Press `ctrl-d` and see
+what happens.
 
 ~~~
 >>> s = input('Enter something --> ')
@@ -33,11 +43,15 @@ Traceback (most recent call last):
 EOFError: EOF when reading a line
 ~~~
 
-Python raises an error called `EOFError` which basically means it found an *end of file* symbol (which is represented by `ctrl-d`) when it did not expect to see it.
+Python raises an error called `EOFError` which basically means it
+found an *end of file* symbol (which is represented by `ctrl-d`) when
+it did not expect to see it.
 
-## Handling Exceptions 
+## Handling Exceptions ##
 
-We can handle exceptions using the `try..except` statement.  We basically put our usual statements within the try-block and put all our error handlers in the except-block.
+We can handle exceptions using the `try..except` statement.  We
+basically put our usual statements within the try-block and put all
+our error handlers in the except-block.
 
 Example (save as `try_except.py`):
 
@@ -70,21 +84,35 @@ You entered no exceptions
 
 How It Works:
 
-We put all the statements that might raise exceptions/errors inside the `try` block and then put handlers for the appropriate errors/exceptions in the `except` clause/block. The `except` clause can handle a single specified error or exception, or a parenthesized list of errors/exceptions. If no names of errors or exceptions are supplied, it will handle *all* errors and exceptions.
+We put all the statements that might raise exceptions/errors inside
+the `try` block and then put handlers for the appropriate
+errors/exceptions in the `except` clause/block. The `except` clause
+can handle a single specified error or exception, or a parenthesized
+list of errors/exceptions. If no names of errors or exceptions are
+supplied, it will handle *all* errors and exceptions.
 
-Note that there has to be at least one `except` clause associated with every `try` clause. Otherwise, what's the point of having a try block?
+Note that there has to be at least one `except` clause associated with
+every `try` clause. Otherwise, what's the point of having a try block?
 
-If any error or exception is not handled, then the default Python handler is called which just stops the execution of the program and prints an error message. We have already seen this in action above.
+If any error or exception is not handled, then the default Python
+handler is called which just stops the execution of the program and
+prints an error message. We have already seen this in action above.
 
-You can also have an `else` clause associated with a `try..except` block. The `else` clause is executed if no exception occurs.
+You can also have an `else` clause associated with a `try..except`
+block. The `else` clause is executed if no exception occurs.
 
-In the next example, we will also see how to get the exception object so that we can retrieve additional information.
+In the next example, we will also see how to get the exception object
+so that we can retrieve additional information.
 
-## Raising Exceptions
+## Raising Exceptions ##
 
-You can *raise* exceptions using the `raise` statement by providing the name of the error/exception and the exception object that is to be *thrown*.
+You can *raise* exceptions using the `raise` statement by providing
+the name of the error/exception and the exception object that is to be
+*thrown*.
 
-The error or exception that you can raise should be a class which directly or indirectly must be a derived class of the `Exception` class.
+The error or exception that you can raise should be a class which
+directly or indirectly must be a derived class of the `Exception`
+class.
 
 Example (save as `raising.py`):
 
@@ -124,13 +152,26 @@ No exception was raised.
 
 How It Works:
 
-Here, we are creating our own exception type. This new exception type is called `ShortInputException`. It has two fields - `length` which is the length of the given input, and `atleast` which is the minimum length that the program was expecting.
+Here, we are creating our own exception type. This new exception type
+is called `ShortInputException`. It has two fields - `length` which is
+the length of the given input, and `atleast` which is the minimum
+length that the program was expecting.
 
-In the `except` clause, we mention the class of error which will be stored `as` the variable name to hold the corresponding error/exception object. This is analogous to parameters and arguments in a function call. Within this particular `except` clause, we use the`length` and `atleast` fields of the exception object to print an appropriate message to the user.
+In the `except` clause, we mention the class of error which will be
+stored `as` the variable name to hold the corresponding
+error/exception object. This is analogous to parameters and arguments
+in a function call. Within this particular `except` clause, we use
+the`length` and `atleast` fields of the exception object to print an
+appropriate message to the user.
 
-## Try .. Finally 
+## Try .. Finally ##
 
-Suppose you are reading a file in your program. How do you ensure that the file object is closed properly whether or not an exception was raised? This can be done using the `finally` block. Note that you can use an `except` clause along with a `finally` block for the same corresponding `try` block. You will have to embed one within another if you want to use both.
+Suppose you are reading a file in your program. How do you ensure that
+the file object is closed properly whether or not an exception was
+raised? This can be done using the `finally` block. Note that you can
+use an `except` clause along with a `finally` block for the same
+corresponding `try` block. You will have to embed one within another
+if you want to use both.
 
 Save as `finally.py`:
 
@@ -165,13 +206,22 @@ if you wanna make your work also fun:
 
 How It Works:
 
-We do the usual file-reading stuff, but we have arbitrarily introduced sleeping for 2 seconds after printing each line using the `time.sleep` function so that the program runs slowly (Python is very fast by nature). When the program is still running, press `ctrl-c` to interrupt/cancel the program.
+We do the usual file-reading stuff, but we have arbitrarily introduced
+sleeping for 2 seconds after printing each line using the `time.sleep`
+function so that the program runs slowly (Python is very fast by
+nature). When the program is still running, press `ctrl-c` to
+interrupt/cancel the program.
 
-Observe that the `KeyboardInterrupt` exception is thrown and the program quits. However, before the program exits, the finally clause is executed and the file object is always closed.
+Observe that the `KeyboardInterrupt` exception is thrown and the
+program quits. However, before the program exits, the finally clause
+is executed and the file object is always closed.
 
-## The with statement 
+## The with statement ##
 
-Acquiring a resource in the `try` block and subsequently releasing the resource in the `finally` block is a common pattern. Hence, there is also a `with` statement that enables this to be done in a clean manner:
+Acquiring a resource in the `try` block and subsequently releasing the
+resource in the `finally` block is a common pattern. Hence, there is
+also a `with` statement that enables this to be done in a clean
+manner:
 
 Save as `using_with.py`:
 
@@ -183,18 +233,32 @@ with open("poem.txt") as f:
 
 How It Works:
 
-The output should be same as the previous example. The difference here is that we are using the `open` function with the `with` statement - we leave the closing of the file to be done automatically by `with open`.
+The output should be same as the previous example. The difference here
+is that we are using the `open` function with the `with` statement -
+we leave the closing of the file to be done automatically by `with
+open`.
 
-What happens behind the scenes is that there is a protocol used by the `with` statement. It fetches the object returned by the `open` statement, let's call it "thefile" in this case.
+What happens behind the scenes is that there is a protocol used by the
+`with` statement. It fetches the object returned by the `open`
+statement, let's call it "thefile" in this case.
 
-It *always* calls the `thefile.__enter__` function before starting the block of code under it and *always* calls `thefile.__exit__` after finishing the block of code.
+It *always* calls the `thefile.__enter__` function before starting the
+block of code under it and *always* calls `thefile.__exit__` after
+finishing the block of code.
 
-So the code that we would have written in a `finally` block should be taken care of automatically by the `__exit__`method. This is what helps us to avoid having to use explicit `try..finally` statements repeatedly.
+So the code that we would have written in a `finally` block should be
+taken care of automatically by the `__exit__`method. This is what
+helps us to avoid having to use explicit `try..finally` statements
+repeatedly.
 
-More discussion on this topic is beyond scope of this book, so please refer [PEP 343](http://www.python.org/dev/peps/pep-0343/) for a comprehensive explanation.
+More discussion on this topic is beyond scope of this book, so please
+refer [PEP 343](http://www.python.org/dev/peps/pep-0343/) for a
+comprehensive explanation.
 
-## Summary 
+## Summary ##
 
-We have discussed the usage of the `try..except` and `try..finally` statements. We have seen how to create our own exception types and how to raise exceptions as well.
+We have discussed the usage of the `try..except` and `try..finally`
+statements. We have seen how to create our own exception types and how
+to raise exceptions as well.
 
 Next, we will explore the Python Standard Library.

16-standard-library.md

@@ -0,0 +1,178 @@
+# Standard Library #
+
+The Python Standard Library contains a huge number of useful modules
+and is part of every standard Python installation. It is important to
+become familiar with the Python Standard Library since many problems
+can be solved quickly if you are familiar with the range of things
+that these libraries can do.
+
+We will explore some of the commonly used modules in this library. You
+can find complete details for all of the modules in the Python
+Standard Library in the
+['Library Reference' section](http://docs.python.org/3/library/) of
+the documentation that comes with your Python installation.
+
+Let us explore a few useful modules.
+
+Note
+
+:   If you find the topics in this chapter too advanced, you may skip
+    this chapter. However, I highly recommend coming back to this
+    chapter when you are more comfortable with programming using
+    Python.
+
+## sys module ##
+
+The `sys` module contains system-specific functionality. We have
+already seen that the `sys.argv` list contains the command-line
+arguments.
+
+Suppose we want to check the version of the Python command being used
+so that, say, we want to ensure that we are using at least
+version 3. The `sys` module gives us such functionality.
+
+~~~
+$ python3
+>>> import sys
+>>> sys.version_info
+sys.version_info(major=3, minor=3, micro=2, releaselevel='final', serial=0)
+>>> sys.version_info.major >= 3
+True
+~~~
+
+How It Works:
+
+The `sys` module has a `version_info` tuple that gives us the version
+information. The first entry is the major version. We can check this
+to, for example, ensure the program runs only under Python 3.0:
+
+Save as `versioncheck.py`:
+
+~~~python
+import sys, warnings
+if sys.version_info.major < 3:
+    warnings.warn("Need Python 3.0 for this program to run",
+        RuntimeWarning)
+else:
+    print('Proceed as normal')
+~~~
+
+Output:
+
+~~~
+$ python2.7 versioncheck.py
+versioncheck.py:6: RuntimeWarning: Need Python 3.0 for this program to run
+  RuntimeWarning)
+
+$ python3 versioncheck.py
+Proceed as normal
+~~~
+
+How It Works:
+
+We use another module from the standard library called `warnings` that
+is used to display warnings to the end-user. If the Python version
+number is not at least 3, we display a corresponding warning.
+
+## logging module ##
+
+What if you wanted to have some debugging messages or important
+messages to be stored somewhere so that you can check whether your
+program has been running as you would expect it? How do you "store
+somewhere" these messages? This can be achieved using the `logging`
+module.
+
+Save as `use_logging.py`:
+
+~~~python
+import os, platform, logging
+
+if platform.platform().startswith('Windows'):
+    logging_file = os.path.join(os.getenv('HOMEDRIVE'), os.getenv('HOMEPATH'), 'test.log')
+else:
+    logging_file = os.path.join(os.getenv('HOME'), 'test.log')
+
+print("Logging to", logging_file)
+
+logging.basicConfig(
+    level=logging.DEBUG,
+    format='%(asctime)s : %(levelname)s : %(message)s',
+    filename = logging_file,
+    filemode = 'w',
+)
+
+logging.debug("Start of the program")
+logging.info("Doing something")
+logging.warning("Dying now")
+~~~
+
+Output:
+
+~~~
+$ python3 use_logging.py
+Logging to C:\Users\swaroop\test.log
+~~~
+
+If we check the contents of `test.log`, it will look something like
+this:
+
+~~~
+2012-10-26 16:52:41,339 : DEBUG : Start of the program
+2012-10-26 16:52:41,339 : INFO : Doing something
+2012-10-26 16:52:41,339 : WARNING : Dying now
+~~~
+
+How It Works:
+
+We use three modules from the standard library - the `os` module for
+interacting with the operating system, the `platform` module for
+information about the platform i.e. the operating system and the
+`logging` module to *log* information.
+
+First, we check which operating system we are using by checking the
+string returned by `platform.platform()` (for more information, see
+`import platform; help(platform)`). If it is Windows, we figure out
+the home drive, the home folder and the filename where we want to
+store the information. Putting these three parts together, we get the
+full location of the file. For other platforms, we need to know just
+the home folder of the user and we get the full location of the file.
+
+We use the `os.path.join()` function to put these three parts of the
+location together. The reason to use a special function rather than
+just adding the strings together is because this function will ensure
+the full location matches the format expected by the operating system.
+
+We configure the `logging` module to write all the messages in a
+particular format to the file we have specified.
+
+Finally, we can put messages that are either meant for debugging,
+information, warning or even critical messages. Once the program has
+run, we can check this file and we will know what happened in the
+program, even though no information was displayed to the user running
+the program.
+
+## Module of the Week Series ##
+
+There is much more to be explored in the standard library such as
+[debugging](http://docs.python.org/3/library/pdb.html),
+[handling command line options](http://docs.python.org/3/library/argparse.html),
+[regular expressions](http://docs.python.org/3/library/re.html) and so
+on.
+
+The best way to further explore the standard library is to read Doug
+Hellmann's excellent
+[Python Module of the Week](http://pymotw.com/2/contents.html) series
+(also available as a
+[book](http://doughellmann.com/python-standard-library-by-example))
+and reading the [Python documentation](http://docs.python.org/3/).
+
+## Summary ##
+
+We have explored some of the functionality of many modules in the
+Python Standard Library. It is highly recommended to browse through
+the
+[Python Standard Library documentation](http://docs.python.org/3/library/)
+to get an idea of all the modules that are available.
+
+Next, we will cover various aspects of Python that will make our tour
+of Python more *complete*.

16-standard-library.pd

@@ -1,123 +0,0 @@
-# Standard Library
-
-The Python Standard Library contains a huge number of useful modules and is part of every standard Python installation. It is important to become familiar with the Python Standard Library since many problems can be solved quickly if you are familiar with the range of things that these libraries can do.
-
-We will explore some of the commonly used modules in this library. You can find complete details for all of the modules in the Python Standard Library in the ['Library Reference' section](http://docs.python.org/py3k/library/index.html) of the documentation that comes with your Python installation.
-
-Let us explore a few useful modules.
-
-Note
-
-:   If you find the topics in this chapter too advanced, you may skip this chapter. However, I highly recommend coming back to this chapter when you are more comfortable with programming using Python.
-
-## sys module 
-
-The `sys` module contains system-specific functionality. We have already seen that the `sys.argv` list contains the command-line arguments.
-
-Suppose we want to check the version of the Python command being used so that, say, we want to ensure that we are using at least version 3. The `sys` module gives us such functionality.
-
-~~~
-$ python3
->>> import sys
->>> sys.version_info
-sys.version_info(major=3, minor=3, micro=0, releaselevel='final', serial=0)
->>> sys.version_info.major >= 3
-True
-~~~
-
-How It Works:
-
-The `sys` module has a `version_info` tuple that gives us the version information. The first entry is the major version. We can check this to, for example, ensure the program runs only under Python 3.0:
-
-Save as `versioncheck.py`:
-
-~~~python
-import sys, warnings
-if sys.version_info.major < 3:
-    warnings.warn("Need Python 3.0 for this program to run",
-        RuntimeWarning)
-else:
-    print('Proceed as normal')
-~~~
-
-Output:
-
-~~~
-$ python2.7 versioncheck.py
-versioncheck.py:6: RuntimeWarning: Need Python 3.0 for this program to run
-  RuntimeWarning)
-
-$ python3 versioncheck.py
-Proceed as normal
-~~~
-
-How It Works:
-
-We use another module from the standard library called `warnings` that is used to display warnings to the end-user. If the Python version number is not at least 3, we display a corresponding warning.
-
-## logging module 
-
-What if you wanted to have some debugging messages or important messages to be stored somewhere so that you can check whether your program has been running as you would expect it? How do you "store somewhere" these messages? This can be achieved using the `logging` module.
-
-Save as `use_logging.py`:
-
-~~~python
-import os, platform, logging
-
-if platform.platform().startswith('Windows'):
-    logging_file = os.path.join(os.getenv('HOMEDRIVE'), os.getenv('HOMEPATH'), 'test.log')
-else:
-    logging_file = os.path.join(os.getenv('HOME'), 'test.log')
-
-print("Logging to", logging_file)
-
-logging.basicConfig(
-    level=logging.DEBUG,
-    format='%(asctime)s : %(levelname)s : %(message)s',
-    filename = logging_file,
-    filemode = 'w',
-)
-
-logging.debug("Start of the program")
-logging.info("Doing something")
-logging.warning("Dying now")
-~~~
-
-Output:
-
-~~~
-$ python3 use_logging.py
-Logging to C:\Users\swaroop\test.log
-~~~
-
-If we check the contents of `test.log`, it will look something like this:
-
-~~~
-2012-10-26 16:52:41,339 : DEBUG : Start of the program
-2012-10-26 16:52:41,339 : INFO : Doing something
-2012-10-26 16:52:41,339 : WARNING : Dying now
-~~~
-
-How It Works:
-
-We use three modules from the standard library - the `os` module for interacting with the operating system, the `platform` module for information about the platform i.e. the operating system and the `logging` module to *log* information.
-
-First, we check which operating system we are using by checking the string returned by `platform.platform()` (for more information, see `import platform; help(platform)`). If it is Windows, we figure out the home drive, the home folder and the filename where we want to store the information. Putting these three parts together, we get the full location of the file. For other platforms, we need to know just the home folder of the user and we get the full location of the file.
-
-We use the `os.path.join()` function to put these three parts of the location together. The reason to use a special function rather than just adding the strings together is because this function will ensure the full location matches the format expected by the operating system.
-
-We configure the `logging` module to write all the messages in a particular format to the file we have specified.
-
-Finally, we can put messages that are either meant for debugging, information, warning or even critical messages. Once the program has run, we can check this file and we will know what happened in the program, even though no information was displayed to the user running the program.
-
-## Module of the Week Series 
-
-There is much more to be explored in the standard library such as [debugging](http://docs.python.org/py3k/library/pdb.html), [handling command line options](http://docs.python.org/py3k/library/argparse.html), [regular expressions](http://docs.python.org/py3k/library/re.html) and so on.
-
-The best way to further explore the standard library is to read Doug Hellmann's excellent [Python Module of the Week](http://www.doughellmann.com/projects/PyMOTW/) series or reading the [Python documentation](http://docs.python.org/py3k/).
-
-## Summary 
-
-We have explored some of the functionality of many modules in the Python Standard Library. It is highly recommended to browse through the [Python Standard Library documentation](http://docs.python.org/py3k/library/index.html) to get an idea of all the modules that are available.
-
-Next, we will cover various aspects of Python that will make our tour of Python more *complete*.

17-more.md

@@ -0,0 +1,285 @@
+# More #
+
+So far we have covered a majority of the various aspects of Python
+that you will use. In this chapter, we will cover some more aspects
+that will make our knowledge of Python more well-rounded.
+
+## Passing tuples around ##
+
+Ever wished you could return two different values from a function? You
+can. All you have to do is use a tuple.
+
+~~~
+>>> def get_error_details():
+...     return (2, 'second error details')
+...
+>>> errnum, errstr = get_error_details()
+>>> errnum
+2
+>>> errstr
+'second error details'
+~~~
+
+Notice that the usage of `a, b = <some expression>` interprets the
+result of the expression as a tuple with two values.
+
+If you want to interpret the results as `(a, <everything else>)`, then
+you just need to star it just like you would in function parameters:
+
+~~~
+>>> a, *b = [1, 2, 3, 4]
+>>> a
+1
+>>> b
+[2, 3, 4]
+~~~
+
+This also means the fastest way to swap two variables in Python is:
+
+~~~
+>>> a = 5; b = 8
+>>> a, b = b, a
+>>> a, b
+(8, 5)
+~~~
+
+## Special Methods ##
+
+There are certain methods such as the `__init__` and `__del__` methods
+which have special significance in classes.
+
+Special methods are used to mimic certain behaviors of built-in
+types. For example, if you want to use the `x[key]` indexing operation
+for your class (just like you use it for lists and tuples), then all
+you have to do is implement the `__getitem__()` method and your job is
+done. If you think about it, this is what Python does for the `list`
+class itself!
+
+Some useful special methods are listed in the following table. If you
+want to know about all the special methods,
+[see the manual](http://docs.python.org/3/reference/datamodel.html#special-method-names).
+
+`__init__(self, ...)`
+
+:   This method is called just before the newly created object is
+    returned for usage.
+
+`__del__(self)`
+
+:   Called just before the object is destroyed
+
+`__str__(self)`
+
+:   Called when we use the `print` function or when `str()`is used.
+
+`__lt__(self, other)`
+
+:   Called when the *less than* operator (&lt;) is used. Similarly,
+    there are special methods for all the operators (+, &gt;, etc.)
+
+`__getitem__(self, key)`
+
+:   Called when `x[key]` indexing operation is used.
+
+`__len__(self)`
+
+:   Called when the built-in `len()` function is used for the sequence
+    object.
+
+## Single Statement Blocks ##
+
+We have seen that each block of statements is set apart from the rest
+by its own indentation level. Well, there is one caveat. If your block
+of statements contains only one single statement, then you can specify
+it on the same line of, say, a conditional statement or looping
+statement. The following example should make this clear:
+
+~~~
+>>> flag = True
+>>> if flag: print('Yes')
+Yes
+~~~
+
+Notice that the single statement is used in-place and not as a
+separate block.  Although, you can use this for making your program
+*smaller*, I strongly recommend avoiding this short-cut method, except
+for error checking, mainly because it will be much easier to add an
+extra statement if you are using proper indentation.
+
+## Lambda Forms ##
+
+A `lambda` statement is used to create new function
+objects. Essentially, the `lambda` takes a parameter followed by a
+single expression only which becomes the body of the function and the
+value of this expression is returned by the new function.
+
+Example (save as `lambda.py`):
+
+~~~python
+points = [ { 'x' : 2, 'y' : 3 }, { 'x' : 4, 'y' : 1 } ]
+points.sort(key=lambda i : i['y'])
+print(points)
+~~~
+
+Output:
+
+~~~
+[{'x': 4, 'y': 1}, {'x': 2, 'y': 3}]
+~~~
+
+How It Works:
+
+Notice that the `sort` method of a `list` can take a `key` parameter
+which determines how the list is sorted (usually we know only about
+ascending or descending order). In our case, we want to do a custom
+sort, and for that we need to write a function but instead of writing
+a separate `def` block for a function that will get used in only this
+one place, we use a lambda expression to create a new function.
+
+## List Comprehension ##
+
+List comprehensions are used to derive a new list from an existing
+list. Suppose you have a list of numbers and you want to get a
+corresponding list with all the numbers multiplied by 2 only when the
+number itself is greater than 2. List comprehensions are ideal for
+such situations.
+
+Example (save as `list_comprehension.py`):
+
+~~~python
+listone = [2, 3, 4]
+listtwo = [2*i for i in listone if i > 2]
+print(listtwo)
+~~~
+
+Output:
+
+~~~
+$ python3 list_comprehension.py
+[6, 8]
+~~~
+
+How It Works:
+
+Here, we derive a new list by specifying the manipulation to be done
+(`2*i`) when some condition is satisfied (`if i > 2`). Note that the
+original list remains unmodified.
+
+The advantage of using list comprehensions is that it reduces the
+amount of boilerplate code required when we use loops to process each
+element of a list and store it in a new list.
+
+## Receiving Tuples and Dictionaries in Functions ##
+
+There is a special way of receiving parameters to a function as a
+tuple or a dictionary using the * or ** prefix respectively. This is
+useful when taking variable number of arguments in the function.
+
+~~~
+>>> def powersum(power, *args):
+...     '''Return the sum of each argument raised to        specified power.'''
+...     total = 0
+...     for i in args:
+...         total += pow(i, power)
+...     return total
+...
+>>> powersum(2, 3, 4)
+25
+
+>>> powersum(2, 10)
+100
+~~~
+
+Because we have a `*` prefix on the `args` variable, all extra
+arguments passed to the function are stored in `args` as a tuple.  If
+a ** prefix had been used instead, the extra parameters would be
+considered to be key/value pairs of a dictionary.
+
+## The assert statement ##
+
+The `assert` statement is used to assert that something is true. For
+example, if you are very sure that you will have at least one element
+in a list you are using and want to check this, and raise an error if
+it is not true, then `assert` statement is ideal in this
+situation. When the assert statement fails, an `AssertionError` is
+raised.
+
+~~~
+>>> mylist = ['item']
+>>> assert len(mylist) >= 1
+>>> mylist.pop()
+'item'
+>>> mylist
+[]
+>>> assert len(mylist) >= 1
+Traceback (most recent call last):
+File "<stdin>", line 1, in <module>
+AssertionError
+~~~
+
+The `assert` statement should be used judiciously. Most of the time,
+it is better to catch exceptions, either handle the problem or display
+an error message to the user and then quit.
+
+## Escape Sequences ##
+
+Suppose, you want to have a string which contains a single quote
+(`'`), how will you specify this string? For example, the string is
+`What's your name?`. You cannot specify `'What's your name?'` because
+Python will be confused as to where the string starts and ends. So,
+you will have to specify that this single quote does not indicate the
+end of the string. This can be done with the help of what is called an
+*escape sequence*. You specify the single quote as `\'` - notice the
+backslash. Now, you can specify the string as `'What\'s your name?'`.
+
+Another way of specifying this specific string would be `"What's your
+name?"` i.e. using double quotes. Similarly, you have to use an escape
+sequence for using a double quote itself in a double quoted
+string. Also, you have to indicate the backslash itself using the
+escape sequence `\\`.
+
+What if you wanted to specify a two-line string? One way is to use a
+triple-quoted string as shown [previously](#triple-quotes) or you can
+use an escape sequence for the newline character - `\n` to indicate
+the start of a new line. An example is `This is the first line\nThis
+is the second line`. Another useful escape sequence to know is the
+tab - `\t`. There are many more escape sequences but I have mentioned
+only the most useful ones here.
+
+One thing to note is that in a string, a single backslash at the end
+of the line indicates that the string is continued in the next line,
+but no newline is added. For example:
+
+~~~python
+"This is the first sentence. \
+This is the second sentence."
+~~~
+
+is equivalent to 
+
+~~~python
+"This is the first sentence. This is the second sentence."
+~~~
+
+### Raw String ###
+
+If you need to specify some strings where no special processing such
+as escape sequences are handled, then what you need is to specify a
+*raw* string by prefixing `r` or `R` to the string. An example is
+`r"Newlines are indicated by \n"`.
+
+Note for Regular Expression Users
+
+:   Always use raw strings when dealing with regular
+    expressions. Otherwise, a lot of backwhacking may be required. For
+    example, backreferences can be referred to as `'\\1'` or `r'\1'`.
+
+## Summary ##
+
+We have covered some more features of Python in this chapter and yet
+we haven't covered all the features of Python. However, at this stage,
+we have covered most of what you are ever going to use in
+practice. This is sufficient for you to get started with whatever
+programs you are going to create.
+
+Next, we will discuss how to explore Python further.

18-what-next.md

@@ -0,0 +1,227 @@
+# What Next #
+
+If you have read this book thoroughly till now and practiced writing a
+lot of programs, then you must have become comfortable and familiar
+with Python. You have probably created some Python programs to try out
+stuff and to exercise your Python skills as well. If you have not done
+it already, you should. The question now is 'What Next?'.
+
+I would suggest that you tackle this problem:
+
+> Create your own command-line *address-book* program using which you
+> can browse, add, modify, delete or search for your contacts such as
+> friends, family and colleagues and their information such as email
+> address and/or phone number. Details must be stored for later
+> retrieval.
+
+This is fairly easy if you think about it in terms of all the various
+stuff that we have come across till now. If you still want directions
+on how to proceed, then here's a hint.
+
+Hint (Don't read without trying first)
+
+:   Create a class to represent the person's information. Use a
+    dictionary to store person objects with their name as the key. Use
+    the pickle module to store the objects persistently on your hard
+    disk. Use the dictionary built-in methods to add, delete and
+    modify the persons.
+
+Once you are able to do this, you can claim to be a Python
+programmer. Now, immediately
+[send me an email](http://swaroopch.com/contact/) thanking me for this
+great book ;-). This step is optional but recommended. Also, please
+consider [buying a printed copy](http://swaroopch.com/buybook/) to
+support the continued development of this book.
+
+If you found that program easy, here's another one:
+
+> Implement the
+> [replace command](http://unixhelp.ed.ac.uk/CGI/man-cgi?replace). This
+> command will replace one string with another in the list of files
+> provided.
+
+The replace command can be as simple or as sophisticated as you wish,
+from simple string substitution to looking for patterns (regular
+expressions).
+
+After that, below are some ways to continue your journey with Python:
+
+## Example Code ##
+
+The best way to learn a programming language is to write a lot of code
+and read a lot of code:
+
+- [Python Cookbook](http://code.activestate.com/recipes/langs/python/)
+  is an extremely valuable collection of recipes or tips on how to
+  solve certain kinds of problems using Python. This is a must-read
+  for every Python user.
+- [Python Module of the Week](http://pymotw.com/2/contents.html) is
+  another excellent must-read guide to the
+  [Python Standard Library](#standard-library).
+
+## Questions and Answers ##
+
+- [Official Python Dos and Don'ts](http://docs.python.org/3/howto/doanddont.html)
+- [Official Python FAQ](http://www.python.org/doc/faq/general/)
+- [Norvig's list of Infrequently Asked Questions](http://norvig.com/python-iaq.html)
+- [Python Interview Q & A](http://dev.fyicenter.com/Interview-Questions/Python/index.html)
+- [StackOverflow questions tagged with python](http://beta.stackoverflow.com/questions/tagged/python)
+
+## Tutorials ##
+
+- [Awaretek's comprehensive list of Python tutorials](http://www.awaretek.com/tutorials.html)
+
+## Videos ##
+
+- [PyVideo](http://www.pyvideo.org/category)
+
+## Discussion ##
+
+If you are stuck with a Python problem, and don't know whom to ask,
+then the
+[python-tutor list](http://mail.python.org/mailman/listinfo/tutor) is
+the best place to ask your question.
+
+Make sure you do your homework and have tried solving the problem
+yourself first.
+
+## News ##
+
+If you want to learn what is the latest in the world of Python, then
+follow the [Official Python Planet](http://planet.python.org).
+
+## Installing libraries ##
+
+There are a huge number of open source libraries at the
+[Python Package Index](http://pypi.python.org/pypi) which you can use
+in your own programs.
+
+To install and use these libraries, you can use
+[pip](http://www.pip-installer.org/en/latest/).
+
+## Graphical Software ##
+
+Suppose you want to create your own graphical programs using
+Python. This can be done using a GUI (Graphical User Interface)
+library with their Python bindings. Bindings are what allow you to
+write programs in Python and use the libraries which are themselves
+written in C or C++ or other languages.
+
+There are lots of choices for GUI using Python:
+
+Kivy
+
+:   <http://kivy.org>
+
+PyGTK
+
+:   This is the Python binding for the GTK+ toolkit which is the
+    foundation upon which GNOME is built. GTK+ has many quirks in
+    usage but once you become comfortable, you can create GUI apps
+    fast. The Glade graphical interface designer is indispensable. The
+    documentation is yet to improve. GTK+ works well on Linux but its
+    port to Windows is incomplete. You can create both free as well as
+    proprietary software using GTK+. To get started, read the
+    [PyGTK tutorial](http://www.pygtk.org/tutorial.html).
+
+PyQt
+
+:   This is the Python binding for the Qt toolkit which is the
+    foundation upon which the KDE is built. Qt is extremely easy to
+    use and very powerful especially due to the Qt Designer and the
+    amazing Qt documentation. PyQt is free if you want to create open
+    source (GPL'ed) software and you need to buy it if you want to
+    create proprietary closed source software. Starting with Qt 4.5
+    you can use it to create non-GPL software as well. To get started,
+    read about [PySide](http://qt-project.org/wiki/PySide).
+
+wxPython
+
+:   This is the Python bindings for the wxWidgets toolkit. wxPython
+    has a learning curve associated with it. However, it is very
+    portable and runs on Linux, Windows, Mac and even embedded
+    platforms. There are many IDEs available for wxPython which
+    include GUI designers as well such as
+    [SPE (Stani's Python Editor)](http://spe.pycs.net/) and the
+    [wxGlade](http://wxglade.sourceforge.net/) GUI builder. You can
+    create free as well as proprietary software using wxPython. To get
+    started, read the
+    [wxPython tutorial](http://zetcode.com/wxpython/).
+
+### Summary of GUI Tools ###
+
+For more choices, see the
+[GuiProgramming wiki page at the official python website](http://www.python.org/cgi-bin/moinmoin/GuiProgramming).
+
+Unfortunately, there is no one standard GUI tool for Python. I suggest
+that you choose one of the above tools depending on your
+situation. The first factor is whether you are willing to pay to use
+any of the GUI tools. The second factor is whether you want the
+program to run only on Windows or on Mac and Linux or all of them. The
+third factor, if Linux is a chosen platform, is whether you are a KDE
+or GNOME user on Linux.
+
+For a more detailed and comprehensive analysis, see Page 26 of the
+[The Python Papers, Volume 3, Issue 1](http://archive.pythonpapers.org/ThePythonPapersVolume3Issue1.pdf).
+
+## Various Implementations ##
+
+There are usually two parts a programming language - the language and
+the software. A language is *how* you write something. The software is
+*what* actually runs our programs.
+
+We have been using the *CPython* software to run our programs. It is
+referred to as CPython because it is written in the C language and is
+the *Classical Python interpreter*.
+
+There are also other software that can run your Python programs:
+
+[Jython](http://www.jython.org)
+
+:   A Python implementation that runs on the Java platform. This means
+    you can use Java libraries and classes from within Python language
+    and vice-versa.
+
+[IronPython](http://www.codeplex.com/Wiki/View.aspx?ProjectName=IronPython)
+
+:   A Python implementation that runs on the .NET platform. This means
+    you can use .NET libraries and classes from within Python language
+    and vice-versa.
+
+[PyPy](http://codespeak.net/pypy/dist/pypy/doc/home.html)
+
+:   A Python implementation written in Python! This is a research
+    project to make it fast and easy to improve the interpreter since
+    the interpreter itself is written in a dynamic language (as
+    opposed to static languages such as C, Java or C# in the above
+    three implementations)
+
+There are also others such as
+[CLPython](http://common-lisp.net/project/clpython/) - a Python
+implementation written in Common Lisp and
+[IronMonkey](http://wiki.mozilla.org/Tamarin:IronMonkey) which is a
+port of IronPython to work on top of a JavaScript interpreter which
+could mean that you can use Python (instead of JavaScript) to write
+your web-browser ("Ajax") programs.
+
+Each of these implementations have their specialized areas where they
+are useful.
+
+## Functional Programming (for advanced readers) ##
+
+When you start writing larger programs, you should definitely learn
+more about a functional approach to programming as opposed to the
+class-based approach to programming that we learned in the
+[object-oriented programming chapter](#object-oriented-programming):
+
+- [Functional Programming Howto by A.M. Kuchling](http://docs.python.org/3/howto/functional.html)
+- [Functional programming chapter in 'Dive Into Python' book](http://www.diveintopython.net/functional_programming/index.html)
+- [Functional Programming with Python presentation](http://ua.pycon.org/static/talks/kachayev/index.html)
+
+## Summary ##
+
+We have now come to the end of this book but, as they say, this is the
+*the beginning of the end*!. You are now an avid Python user and you
+are no doubt ready to solve many problems using Python. You can start
+automating your computer to do all kinds of previously unimaginable
+things or write your own games and much much more. So, get started!

19-appendix-floss.md

@@ -0,0 +1,95 @@
+# FLOSS #
+
+Free/Libre and Open Source Software, in short,
+[FLOSS](http://en.wikipedia.org/wiki/FLOSS) is based on the concept of
+a community, which itself is based on the concept of sharing, and
+particularly the sharing of knowledge. FLOSS are free for usage,
+modification and redistribution.
+
+If you have already read this book, then you are already familiar with
+FLOSS since you have been using **Python** all along and Python is an
+open source software!
+
+Here are some examples of FLOSS to give an idea of the kind of things
+that community sharing and building can create:
+
+[Linux](http://www.kernel.org)
+
+:   This is a FLOSS OS kernel used in the GNU/Linux operating
+    system. Linux, the kernel, was started by Linus Torvalds as a
+    student. Android is based on Linux. Any website you use these days
+    will mostly be running on Linux.
+
+[Ubuntu](http://www.ubuntu.com)
+
+:   This is a community-driven distribution, sponsored by Canonical
+    and it is the most popular Linux distribution today. It allows you
+    to install a plethora of FLOSS available and all this in an
+    easy-to-use and easy-to-install manner. Best of all, you can just
+    reboot your computer and run GNU/Linux off the CD! This allows you
+    to completely try out the new OS before installing it on your
+    computer. However, Ubuntu is not entirely free software; it
+    contains proprietary drivers, firmware, and applications.
+
+[LibreOffice](http://www.libreoffice.org/)
+
+:   This is an excellent community-driven and developed office suite
+    with a writer, presentation, spreadsheet and drawing components
+    among other things. It can even open and edit MS Word and MS
+    PowerPoint files with ease. It runs on almost all platforms and is
+    entirely free, libre and open source software.
+
+[Mozilla Firefox](http://www.mozilla.org/products/firefox)
+
+:   This is *the* best web browser. It is blazingly fast and has
+    gained critical acclaim for its sensible and impressive
+    features. The extensions concept allows any kind of plugins to be
+    used.
+
+    Its companion product
+    [Thunderbird](http://www.mozilla.org/products/thunderbird) is an
+    excellent email client that makes reading email a snap.
+
+[Mono](http://www.mono-project.com)
+
+:   This is an open source implementation of the Microsoft .NET
+    platform.  It allows .NET applications to be created and run on
+    GNU/Linux, Windows, FreeBSD, Mac OS and many other platforms as
+    well.
+
+[Apache web server](http://httpd.apache.org)
+
+:   This is the popular open source web server. In fact, it is *the*
+    most popular web server on the planet! It runs nearly more than
+    half of the websites out there. Yes, that's right - Apache handles
+    more websites than all the competition (including Microsoft IIS)
+    combined.
+
+[VLC Player](http://www.videolan.org/vlc/)
+
+:   This is a video player that can play anything from DivX to MP3 to
+    Ogg to VCDs and DVDs to ... who says open source ain't fun? ;-)
+
+This list is just intended to give you a brief idea - there are many
+more excellent FLOSS out there, such as the Perl language, PHP
+language, Drupal content management system for websites, PostgreSQL
+database server, TORCS racing game, KDevelop IDE, Xine - the movie
+player, VIM editor, Quanta+ editor, Banshee audio player, GIMP image
+editing program, ... This list could go on forever.
+
+To get the latest buzz in the FLOSS world, check out the following
+websites:
+
+- [OMG! Ubuntu!](http://www.omgubuntu.co.uk/)
+- [Web Upd8](http://www.webupd8.org/)
+- [DistroWatch](http://www.distrowatch.com)
+- [Planet Debian](http://planet.debian.org/)
+
+Visit the following websites for more information on FLOSS:
+
+- [GitHub Explore](http://github.com/explore)
+- [Code Triage](http://www.codetriage.com/)
+- [SourceForge](http://www.sourceforge.net)
+- [FreshMeat](http://www.freshmeat.net)
+
+So, go ahead and explore the vast, free and open world of FLOSS!

20-appendix-about.md

@@ -0,0 +1,52 @@
+# Colophon #
+
+Almost all of the software that I have used in the creation of this
+book are [FLOSS](#floss).
+
+## Birth of the Book ##
+
+In the first draft of this book, I had used Red Hat 9.0 Linux as the
+foundation of my setup and in the sixth draft, I used Fedora Core 3
+Linux as the basis of my setup.
+
+Initially, I was using KWord to write the book (as explained in the
+[history lesson](#history-lesson) in the preface).
+
+## Teenage Years ##
+
+Later, I switched to DocBook XML using Kate but I found it too
+tedious. So, I switched to OpenOffice which was just excellent with
+the level of control it provided for formatting as well as the PDF
+generation, but it produced very sloppy HTML from the document.
+
+Finally, I discovered XEmacs and I rewrote the book from scratch in
+DocBook XML (again) after I decided that this format was the long term
+solution.
+
+In the sixth draft, I decided to use Quanta+ to do all the
+editing. The standard XSL stylesheets that came with Fedora Core 3
+Linux were being used. However, I had written a CSS document to give
+color and style to the HTML pages. I had also written a crude lexical
+analyzer, in Python of course, which automatically provides syntax
+highlighting to all the program listings.
+
+For the seventh draft, I'm using [MediaWiki](http://www.mediawiki.org)
+as the basis of my setup. I used to edit everything online and the
+readers can directly read/edit/discuss within the wiki website, but I
+ended up spending more time fighting spam than writing.
+
+## Now ##
+
+For the eight draft, I used [Vim](http://www.swaroopch.com/notes/vim),
+[Pandoc](http://johnmacfarlane.net/pandoc/README.html), and Mac OS X.
+
+Now I use
+[Emacs 24.3](http://www.masteringemacs.org/articles/2013/03/11/whats-new-emacs-24-3/) +
+[ghoseb's dotemacs](https://github.com/ghoseb/dotemacs) +
+[Jason Blevins' Markdown Mode](http://jblevins.org/projects/markdown-mode/)
+which is a beautiful combination of software. As you may have
+realized, I am quite happy with this editing setup.
+
+## About The Author ##
+
+<http://swaroopch.com/about/>

20-appendix-about.pd

@@ -1,29 +0,0 @@
-# Colophon 
-
-Almost all of the software that I have used in the creation of this book are [FLOSS](#floss).
-
-## Birth of the Book 
-
-In the first draft of this book, I had used Red Hat 9.0 Linux as the foundation of my setup and in the sixth draft, I used Fedora Core 3 Linux as the basis of my setup.
-
-Initially, I was using KWord to write the book (as explained in the [history lesson](#history-lesson) in the preface).
-
-## Teenage Years 
-
-Later, I switched to DocBook XML using Kate but I found it too tedious. So, I switched to OpenOffice which was just excellent with the level of control it provided for formatting as well as the PDF generation, but it produced very sloppy HTML from the document.
-
-Finally, I discovered XEmacs and I rewrote the book from scratch in DocBook XML (again) after I decided that this format was the long term solution.
-
-In the sixth draft, I decided to use Quanta+ to do all the editing. The standard XSL stylesheets that came with Fedora Core 3 Linux were being used. However, I had written a CSS document to give color and style to the HTML pages. I had also written a crude lexical analyzer, in Python of course, which automatically provides syntax highlighting to all the program listings.
-
-For this seventh draft, I'm using [MediaWiki](http://www.mediawiki.org ) as the basis of my [setup](http://www.swaroopch.com/notes/). Now I edit everything online and the readers can directly read/edit/discuss within the wiki website.
-
-I used Vim for editing thanks to the [ViewSourceWith extension for Firefox](https://addons.mozilla.org/en-US/firefox/addon/394) that integrates with Vim.
-
-## Now
-
-Using [Vim](http://www.swaroopch.com/notes/vim), [Pandoc](http://johnmacfarlane.net/pandoc/README.html), and Mac OS X.
-
-## About The Author 
-
-<http://www.swaroopch.com/about/>

21-revision-history.md

@@ -0,0 +1,82 @@
+# Revision History
+
+- 2.1
+    - 03 Aug 2013
+    - Reformatted using Emacs 24 +
+      [Jason Blevins' Markdown Mode](http://jblevins.org/projects/markdown-mode/)
+- 2.0
+    - 20 Oct 2012
+    - Rewritten in
+      [Pandoc format](http://johnmacfarlane.net/pandoc/README.html),
+      thanks to my wife who did most of the conversion from the
+      Mediawiki format
+    - Simplifying text, removing non-essential sections such as
+      `nonlocal` and metaclasses
+- 1.90 
+    - 04 Sep 2008 and still in progress
+    - Revival after a gap of 3.5 years! 
+    - Rewriting for Python 3.0
+    - Rewrite using [MediaWiki](http://www.mediawiki.org) (again)
+- 1.20 
+    - 13 Jan 2005
+    - Complete rewrite using
+      [Quanta+](https://en.wikipedia.org/wiki/Quanta_Plus) on
+      [Fedora](http://fedoraproject.org/) Core 3 with lot of
+      corrections and updates. Many new examples. Rewrote my DocBook
+      setup from scratch.
+- 1.15 
+    - 28 Mar 2004
+    - Minor revisions 
+- 1.12 
+    - 16 Mar 2004
+    - Additions and corrections. 
+- 1.10 
+    - 09 Mar 2004
+    - More typo corrections, thanks to many enthusiastic and helpful
+      readers.
+- 1.00 
+    - 08 Mar 2004
+    - After tremendous feedback and suggestions from readers, I have
+      made significant revisions to the content along with typo
+      corrections.
+- 0.99 
+    - 22 Feb 2004
+    - Added a new chapter on modules. Added details about variable
+      number of arguments in functions.
+- 0.98 
+    - 16 Feb 2004
+    - Wrote a Python script and CSS stylesheet to improve XHTML
+      output, including a crude-yet-functional lexical analyzer for
+      automatic VIM-like syntax highlighting of the program listings.
+- 0.97 
+    - 13 Feb 2004
+    - Another completely rewritten draft, in DocBook XML (again). Book
+      has improved a lot - it is more coherent and readable.
+- 0.93 
+    - 25 Jan 2004
+    - Added IDLE talk and more Windows-specific stuff
+- 0.92 
+    - 05 Jan 2004
+    - Changes to few examples. 
+- 0.91
+    - 30 Dec 2003
+    - Corrected typos. Improvised many topics. 
+- 0.90 
+    - 18 Dec 2003
+    - Added 2 more
+      chapters. [OpenOffice](https://en.wikipedia.org/wiki/OpenOffice)
+      format with revisions.
+- 0.60 
+    - 21 Nov 2003
+    - Fully rewritten and expanded. 
+- 0.20 
+    - 20 Nov 2003
+    - Corrected some typos and errors. 
+- 0.15 
+    - 20 Nov 2003
+    - Converted to
+      [DocBook XML](https://en.wikipedia.org/wiki/DocBook) with
+      XEmacs.
+- 0.10 
+    - 14 Nov 2003
+    - Initial draft using [KWord](https://en.wikipedia.org/wiki/Kword).

21-revision-history.pd

@@ -1,59 +0,0 @@
-# Revision History
-
-- 2.0
-    - 20 Oct 2012
-    - Rewritten in [Pandoc format](http://johnmacfarlane.net/pandoc/README.html), thanks to my wife who did most of the conversion from the Mediawiki format
-    - Simplifying text, removing non-essential sections such as `nonlocal` and metaclasses
-- 1.90 
-    - 04 Sep 2008 and still in progress 
-    - Revival after a gap of 3.5 years! 
-    - Rewriting for Python 3.0
-    - Rewrite using [MediaWiki](http://www.mediawiki.org) (again) 
-- 1.20 
-    - 13 Jan 2005
-    - Complete rewrite using [Quanta+](https://en.wikipedia.org/wiki/Quanta_Plus) on [Fedora](http://fedoraproject.org/) Core 3 with lot of corrections and updates. Many new examples. Rewrote my DocBook setup from scratch. 
-- 1.15 
-    - 28 Mar 2004
-    - Minor revisions 
-- 1.12 
-    - 16 Mar 2004
-    - Additions and corrections. 
-- 1.10 
-    - 09 Mar 2004
-    - More typo corrections, thanks to many enthusiastic and helpful readers. 
-- 1.00 
-    - 08 Mar 2004
-    - After tremendous feedback and suggestions from readers, I have made significant revisions to the content along with typo corrections. 
-- 0.99 
-    - 22 Feb 2004
-    - Added a new chapter on modules. Added details about variable number of arguments in functions. 
-- 0.98 
-    - 16 Feb 2004
-    - Wrote a Python script and CSS stylesheet to improve XHTML output, including a crude-yet-functional lexical analyzer for automatic VIM-like syntax highlighting of the program listings. 
-- 0.97 
-    - 13 Feb 2004
-    - Another completely rewritten draft, in DocBook XML (again). Book has improved a lot - it is more coherent and readable. 
-- 0.93 
-    - 25 Jan 2004
-    - Added IDLE talk and more Windows-specific stuff 
-- 0.92 
-    - 05 Jan 2004
-    - Changes to few examples. 
-- 0.91 
-    - 30 Dec 2003
-    - Corrected typos. Improvised many topics. 
-- 0.90 
-    - 18 Dec 2003
-    - Added 2 more chapters. [OpenOffice](https://en.wikipedia.org/wiki/OpenOffice) format with revisions. 
-- 0.60 
-    - 21 Nov 2003
-    - Fully rewritten and expanded. 
-- 0.20 
-    - 20 Nov 2003
-    - Corrected some typos and errors. 
-- 0.15 
-    - 20 Nov 2003
-    - Converted to [DocBook XML](https://en.wikipedia.org/wiki/DocBook) with XEmacs. 
-- 0.10 
-    - 14 Nov 2003
-    - Initial draft using [KWord](https://en.wikipedia.org/wiki/Kword).

22-translations.md

@@ -0,0 +1,392 @@
+# Translations #
+
+There are many translations of the book available in different human
+languages, thanks to many tireless volunteers!
+
+If you want to help with these translations, please see the list of
+volunteers and languages below and decide if you want to start a new
+translation or help in existing translation projects.
+
+If you plan to start a new translation, please read the
+[Translation Howto](#translation-howto).
+
+## Arabic ##
+ 
+Below is the link for the Arabic version. Thanks to Ashraf Ali Khalaf
+for translating the book, you can read the whole book online
+[here](http://www.khaledhosny.org/byte-of-python/index.html) or you
+can download it from
+[sourceforge.net](http://downloads.sourceforge.net/omlx/byteofpython_arabic.pdf?use_mirror=osdn)
+for more info [click here](http://itwadi.com/byteofpython_arabi).
+
+## Brazilian Portuguese ##
+
+There are two translations:
+
+[Samuel Dias Neto](http://www.samueldiasneto.com/aprendendopython/index.html)
+(samuel.arataca-at-gmail-dot-com) made the first Brazilian Portuguese
+translation of this book when Python was in 2.3.5 version.
+
+Samuel's translation is available at
+[aprendendopython](http://www.samueldiasneto.com/aprendendopython/index.html).
+
+[Rodrigo Amaral](http://rodrigoamaral.net)
+(rodrigoamaral-at-gmail-dot-com) has volunteered to translate the book
+to Brazilian Portuguese.
+
+Rodrigo's translation is available at
+[http://www.swaroopch.org/notes/Python_pt-br:Indice](http://www.swaroopch.org/notes/Python_pt-br:Indice).
+
+## Catalan ##
+
+Moises Gomez (moisesgomezgiron-at-gmail-dot-com) has volunteered to
+translate the book to Catalan.  The translation is in progress, and
+was present in the [erstwhile wiki](http://www.swaroopch.com/notes/).
+
+> Moisès Gómez - I am a developer and also a teacher of programming
+> (normally for people without any previous experience).
+
+> Some time ago I needed to learn how to program in Python, and
+> Swaroop's work was really helpful. Clear, concise, and complete
+> enough. Just what I needed.
+
+> After this experience, I thought some other people in my country
+> could take benefit from it too. But English language can be a
+> barrier.
+
+> So, why not try to translate it? And I did for a previous version of
+> BoP.
+
+> I my country there are two official languages. I selected the
+> Catalan language assuming that others will translate it to the more
+> widespread Spanish.
+
+## Chinese ##
+
+Here's a Chinese version at
+[http://www.swaroopch.org/notes/Python_cn:Table_of_Contents](http://www.swaroopch.org/notes/Python_cn:Table_of_Contents).
+
+Juan Shen (orion-underscore-val-at-163-dot-com) has volunteered to
+translate the book to Chinese.  It is available at
+[http://www.pycn.org/python%E5%9C%A8%E7%BA%BF%E6%89%8B%E5%86%8C](http://www.pycn.org/python%E5%9C%A8%E7%BA%BF%E6%89%8B%E5%86%8C).
+
+> I am a postgraduate at Wireless Telecommunication Graduate School,
+> Beijing University of Technology, China PR. My current research
+> interest is on the synchronization, channel estimation and
+> multi-user detection of multicarrier CDMA system. Python is my major
+> programming language for daily simulation and research job, with the
+> help of Python Numeric, actually. I learned Python just half a year
+> before, but as you can see, it's really easy-understanding,
+> easy-to-use and productive. Just as what is ensured in Swaroop's
+> book, 'It's my favorite programming language now'.
+
+> 'A Byte of Python' is my tutorial to learn Python. It's clear and
+> effective to lead you into a world of Python in the shortest time.
+> It's not too long, but efficiently covers almost all important
+> things in Python. I think 'A Byte of Python' should be strongly
+> recommendable for newbies as their first Python tutorial. Just
+> dedicate my translation to the potential millions of Python users in
+> China.
+
+## Chinese Traditional ##
+
+Fred Lin (gasolin-at-gmail-dot-com) has volunteered to translate the
+book to Chinese Traditional.
+
+It is available at [http://code.google.com/p/zhpy/wiki/ByteOfZhpy](http://code.google.com/p/zhpy/wiki/ByteOfZhpy).
+
+An exciting feature of this translation is that it also contains the
+*executable chinese python sources* side by side with the original
+python sources.
+
+> **Fred Lin** - I'm working as a network firmware engineer at Delta
+> Network, and I'm also a contributor of TurboGears web framework.
+
+> As a python evangelist (:-p), I need some material to promote python
+> language. I found 'A Byte of Python' hit the sweet point for both
+> newbies and experienced programmers. 'A Byte of Python' elaborates
+> the python essentials with affordable size.
+
+> The translation are originally based on simplified chinese version,
+> and soon a lot of rewrite were made to fit the current wiki version
+> and the quality of reading.
+
+> The recent chinese traditional version also featured with executable
+> chinese python sources, which are achieved by my new 'zhpy' (python
+> in chinese) project (launch from Aug 07).
+
+> zhpy(pronounce (Z.H.?, or zippy) build a layer upon python to
+> translate or interact with python in chinese(Traditional or
+> Simplified). This project is mainly aimed for education.
+
+## French ##
+
+Gregory (coulix-at-ozforces-dot-com-dot-au) has volunteered to
+translate the book to French.
+
+Gérard Labadie (Palmipede) has completed to translate the book to
+French, it starts with
+[http://www.swaroopch.org/notes/Python_fr:Table_des_Mati%C3%A8res](http://www.swaroopch.org/notes/Python_fr:Table_des_Mati%C3%A8res).
+
+## German ##
+
+Lutz Horn (lutz-dot-horn-at-gmx-dot-de), Bernd Hengelein
+(bernd-dot-hengelein-at-gmail-dot-com) and Christoph Zwerschke
+(cito-at-online-dot-de) have volunteered to translate the book to
+German.
+
+Their translation is located at
+[http://abop-german.berlios.de](http://abop-german.berlios.de).
+
+*Lutz Horn* says:
+
+> I'm 32 years old and have a degree of Mathematics from University of
+> Heidelberg, Germany. Currently I'm working as a software engineer on
+> a publicly funded project to build a web portal for all things
+> related to computer science in Germany.The main language I use as a
+> professional is Java, but I try to do as much as possible with
+> Python behind the scenes. Especially text analysis and conversion is
+> very easy with Python. I'm not very familiar with GUI toolkits,
+> since most of my programming is about web applications, where the
+> user interface is build using Java frameworks like Struts. Currently
+> I try to make more use of the functional programming features of
+> Python and of generators.  After taking a short look into Ruby, I
+> was very impressed with the use of blocks in this
+> language. Generally I like the dynamic nature of languages like
+> Python and Ruby since it allows me to do things not possible in more
+> static languages like Java.I've searched for some kind of
+> introduction to programming, suitable to teach a complete
+> non-programmer. I've found the book 'How to Think Like a Computer
+> Scientist: Learning with Python', and 'Dive into Python'. The first
+> is good for beginners but to long to translate. The second is not
+> suitable for beginners. I think 'A Byte of Python' falls nicely
+> between these, since it is not too long, written to the point, and
+> at the same time verbose enough to teach a newbie. Besides this, I
+> like the simple DocBook structure, which makes translating the text
+> a generation the output in various formats a charm.
+
+*Bernd Hengelein* says:
+
+> Lutz and me are going to do the german translation together. We just
+> started with the intro and preface but we will keep you informed
+> about the progress we make. Ok, now some personal things about me. I
+> am 34 years old and playing with computers since the 1980's, when
+> the "Commodore C64" ruled the nurseries. After studying computer
+> science I started working as a software engineer. Currently I am
+> working in the field of medical imaging for a major german
+> company. Although C++ is the main language I (have to) use for my
+> daily work, I am constantly looking for new things to learn.Last
+> year I fell in love with Python, which is a wonderful language, both
+> for its possibilities and its beauty. I read somewhere in the net
+> about a guy who said that he likes python, because the code looks so
+> beautiful. In my opinion he's absolutly right. At the time I decided
+> to learn python, I noticed that there is very little good
+> documentation in german available. When I came across your book the
+> spontaneous idea of a german translation crossed my mind. Luckily,
+> Lutz had the same idea and we can now divide the work.I am looking
+> forward to a good cooperation!
+
+## Greek ##
+
+The Greek Ubuntu Community
+[translated the book in Greek](http://wiki.ubuntu-gr.org/byte-of-python-el),
+for use in our on-line asynchronous Python lessons that take place in
+our forums. Contact
+[@savvasradevic](https://twitter.com/savvasradevic) for more
+information.
+
+## Indonesian ##
+
+Daniel (daniel-dot-mirror-at-gmail-dot-com) is translating the book to
+Indonesian at
+[http://python.or.id/moin.cgi/ByteofPython](http://python.or.id/moin.cgi/ByteofPython).
+
+W. Priyambodo also has volunteered to translate the book to
+Indonesian. The translation is in progress and here is the
+[http://www.swaroopch.org/notes/Python_id:Daftar_Isi](http://www.swaroopch.org/notes/Python_id:Daftar_Isi).
+
+## Italian ##
+
+Enrico Morelli (mr-dot-mlucci-at-gmail-dot-com) and Massimo Lucci
+(morelli-at-cerm-dot-unifi-dot-it) have volunteered to translate the
+book to Italian.
+
+The Italian translation is present at
+[www.gentoo.it/Programmazione/byteofpython](http://www.gentoo.it/Programmazione/byteofpython). The
+new translation is in progress and start with
+[http://www.swaroopch.org/notes/Python_it:Prefazione](http://www.swaroopch.org/notes/Python_it:Prefazione).
+
+> **Massimo Lucci and Enrico Morelli** - we are working at the
+> University of Florence (Italy) - Chemistry Department. I (Massimo)
+> as service engineer and system administrator for Nuclear Magnetic
+> Resonance Spectrometers; Enrico as service engineer and system
+> administrator for our CED and parallel / clustered systems. We are
+> programming on python since about seven years, we had experience
+> working with Linux platforms since ten years. In Italy we are
+> responsible and administrator for www.gentoo.it web site for
+> Gentoo/Linux distrubution and www.nmr.it (now under construction)
+> for Nuclear Magnetic Resonance applications and Congress
+> Organization and Managements.That's all! We are impressed by the
+> smart language used on your Book and we think this is essential for
+> approaching the Python to new users (we are thinking about hundred
+> of students and researcher working on our labs).
+
+## Japanese ##
+
+Here's a Japanese version at
+[http://www.swaroopch.org/notes/Python_ja:Table_of_Contents](http://www.swaroopch.org/notes/Python_ja:Table_of_Contents).
+
+Shunro Dozono (dozono-at-gmail-dot-com) is translating the book to
+Japanese.
+
+## Mongolian ##
+
+Ariunsanaa Tunjin (luftballons2010-at-gmail-dot-com) has volunteered
+to translate the book to Mongolian.
+
+_Update on Nov 22, 2009_ : Ariunsanaa is on the verge of completing
+the translation.
+
+## Norwegian (bokmål) ##
+
+Eirik Vågeskar
+([http://www.swaroopch.org/notes/User:Vages](http://www.swaroopch.org/notes/User:Vages))
+is a high school student at
+[Sandvika videregående skole](http://no.wikipedia.org/wiki/Sandvika_videreg%C3%A5ende_skole)
+in Norway, a [blogger](http://forbedre.blogspot.com/) and currently
+translating the book to Norwegian (bokmål). The translation is in
+progress, and you can check the
+[http://www.swaroopch.org/notes/Python_nb-no:Innholdsfortegnelse](http://www.swaroopch.org/notes/Python_nb-no:Innholdsfortegnelse)
+for more details.
+
+> _Eirik Vågeskar_: I have always wanted to program, but because I
+> speak a small language, the learning process was much harder. Most
+> tutorials and books are written in very technical English, so most
+> high school graduates will not even have the vocabulary to
+> understand what the tutorial is about. When I discovered this book,
+> all my problems were solved. "A Byte of Python" used simple
+> non-technical language to explain a programming language that is
+> just as simple, and these two things make learning Python fun. After
+> reading half of the book, I decided that the book was worth
+> translating. I hope the translation will help people who have found
+> themself in the same situation as me (especially young people), and
+> maybe help spread interest for the language among people with less
+> technical knowledge.
+
+## Polish ##
+
+Dominik Kozaczko (dkozaczko-at-gmail-dot-com) has volunteered to
+translate the book to Polish. Translation is in progress and it's main
+page is available here:
+[Ukąś Pythona](http://wiki.lo5.bielsko.pl/index.php/Uk%C4%85%C5%9B_Pythona).
+
+*Update* : The translation is complete and ready as of Oct
+ 2, 2009. Thanks to Dominik, his two students and their friend for
+ their time and effort!
+
+> *Dominik Kozaczko* - I'm a Computer Science and Information
+> Technology teacher.
+
+## Portuguese ##
+
+Fidel Viegas (fidel-dot-viegas-at-gmail-dot-com) has volunteered to
+translate the book to Portuguese.
+
+## Romanian ##
+
+Paul-Sebastian Manole (brokenthorn-at-gmail-dot-com) has volunteered
+to translate this book to Romanian.
+
+> *Paul-Sebastian Manole* - I'm a second year Computer Science student
+> at Spiru Haret University, here in Romania. I'm more of a
+> self-taught programmer and decided to learn a new language,
+> Python. The web told me there was no better way to do so but read
+> ''A Byte of Python''. That's how popular this book is
+> (congratulations to the author for writing such an easy to read
+> book). I started liking Python so I decided to help translate the
+> latest version of Swaroop's book in Romanian. Although I could be
+> the one with the first initiative, I'm just one volunteer so if you
+> can help, please join me.
+
+The translation was being done at
+[http://www.swaroopch.org/notes/Python_ro](http://www.swaroopch.org/notes/Python_ro).
+
+## Russian ##
+
+Vladimir Smolyar (v_2e-at-ukr-dot-net) has completed a Russian
+translation at <http://wombat.org.ua/AByteOfPython/>.
+
+## Ukranian ##
+
+Averkiev Andrey (averkiyev-at-ukr-dot-net) has volunteered to
+translate the book to Russian, and perhaps Ukranian (time permitting).
+
+## Slovak ##
+
+Albertio Ward (albertioward-at-gmail-dot-com) has translated the book
+to Slovak at
+[fatcow.com/edu/python-swaroopch-sl/](http://www.fatcow.com/edu/python-swaroopch-sl/)
+:
+
+> We are a non-profit organization called "Translation for
+> education". We represent a group of people, mainly students and
+> professors, of the Slavonic University. Here are students from
+> different departments: linguistics, chemistry, biology, etc. We try
+> to find interesting publications on the Internet that can be
+> relevant for us and our university colleagues. Sometimes we find
+> articles by ourselves; other times our professors help us choose the
+> material for translation. After obtaining permission from authors we
+> translate articles and post them in our blog which is available and
+> accessible to our colleagues and friends. These translated
+> publications often help students in their daily study routine.
+
+> Why have I chosen your article for translation? It was made to help
+> the Bulgarians understand this article in all possible detail.
+> Having done some research concerning the novelty and relevance of
+> topics, I found that it was quite urgent for those who live in my
+> country. So,I think it can become very popular. The language barrier
+> in this little case no longer exists, because it was eliminated by
+> my translation.
+
+## Spanish ##
+
+Alfonso de la Guarda Reyes (alfonsodg-at-ictechperu-dot-net), Gustavo
+Echeverria (gustavo-dot-echeverria-at-gmail-dot-com), David Crespo
+Arroyo (davidcrespoarroyo-at-hotmail-dot-com) and Cristian Bermudez
+Serna (crisbermud-at-hotmail-dot-com) have volunteered to translate
+the book to Spanish. The translation is in progress, you can read the
+spanish (argentinian) translation starting by the
+[http://www.swaroopch.org/notes/Python_es-ar:Tabla_de_Contenidos](http://www.swaroopch.org/notes/Python_es-ar:Tabla_de_Contenidos).
+
+*Gustavo Echeverria* says:
+
+> I work as a software engineer in Argentina. I use mostly C# and .Net
+> technologies at work but strictly Python or Ruby in my personal
+> projects. I knew Python many years ago and I got stuck
+> inmediately. Not so long after knowing Python I discovered this book
+> and it helped me to learn the language. Then I volunteered to
+> translate the book to Spanish. Now, after receiving some requests,
+> I've begun to translate "A Byte of Python" with the help of
+> Maximiliano Soler.
+
+*Cristian Bermudez Serna* says:
+
+> I am student of Telecommunications engineering at the University of
+> Antioquia (Colombia). Months ago, i started to learn Python and
+> found this wonderful book, so i volunteered to get the Spanish
+> translation.  </blockquote>
+
+## Swedish ##
+
+Mikael Jacobsson (leochingkwake-at-gmail-dot-com) has volunteered to
+translate the book to Swedish.
+
+## Turkish ##
+
+Türker SEZER (tsezer-at-btturk-dot-net) and Bugra Cakir
+(bugracakir-at-gmail-dot-com) have volunteered to translate the book
+to Turkish. Where is Turkish version? Bitse de okusak.
+
+*Note* : Replace `-at-` with `@` , `-dot-` with `.` and `-underscore-`
+ with `_` in the email addresses mentioned on this page. Dashes in
+ other places in the email address remain as-is.

23-translation-howto.md

@@ -0,0 +1,19 @@
+# Translation Howto #
+
+The full source of the book is available from
+<https://github.com/swaroopch/byte_of_python>.
+
+Please
+[fork the repository](https://help.github.com/articles/fork-a-repo).
+
+Then, fetch the repository to your computer. You need to know how to
+use [Git](http://www.git-scm.com) to do that.
+
+Start editing the `.md` files to your local language. Please read the
+[Pandoc README](http://johnmacfarlane.net/pandoc/README.html#pandocs-markdown)
+to understand the formatting of the text.
+
+Then follow the instructions in the
+[README](http://github.com/swaroopch/byte_of_python#README) to install
+the software required to be able to convert the raw source files into
+PDFs, etc.

23-translation-howto.pd

@@ -1,11 +0,0 @@
-# Translation Howto
-
-The full source of the book is available from the Git repository <https://github.com/swaroopch/byte_of_python>.
-
-Please [fork the repository](https://help.github.com/articles/fork-a-repo).
-
-Then, fetch the repository to your computer. You need to know how to use [Git](http://www.git-scm.com) to do that.
-
-Start editing the `.pd` files to your local language. Please read the [Pandoc README](http://johnmacfarlane.net/pandoc/README.html#pandocs-markdown) to understand the formatting of the text.
-
-Then follow the instructions in the [README](http://github.com/swaroopch/byte_of_python#README) to install the software required to be able to convert the raw source files into PDFs, etc.

LICENSE

@@ -1 +1,5 @@
-This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License - see http://creativecommons.org/licenses/by-sa/3.0/deed.en_US for details.
+This work is licensed under the "Creative Commons
+Attribution-ShareAlike 3.0 Unported" License.
+ 
+See <http://creativecommons.org/licenses/by-sa/3.0/deed.en_US> for
+details.

config.json

@@ -3,117 +3,117 @@
   "FULL_PROJECT_NAME": "byte_of_python",
   "MARKDOWN_FILES": [
     {
-      "file": "01-frontpage.pd",
+      "file": "01-frontpage.md",
       "slug": "python",
       "title": "Python"
     },
     {
-      "file": "02-preface.pd",
+      "file": "02-preface.md",
       "slug": "python_en-preface",
       "title": "Python : Preface"
     },
     {
-      "file": "03-intro.pd",
+      "file": "03-intro.md",
       "slug": "python_en-introduction",
       "title": "Python : Introduction"
     },
     {
-      "file": "04-installation.pd",
+      "file": "04-installation.md",
       "slug": "python_en-installation",
       "title": "Python : Installation"
     },
     {
-      "file": "05-first-steps.pd",
+      "file": "05-first-steps.md",
       "slug": "python_en-first_steps",
       "title": "Python : First Steps"
     },
     {
-      "file": "06-basics.pd",
+      "file": "06-basics.md",
       "slug": "python_en-basics",
       "title": "Python : Basics"
     },
     {
-      "file": "07-operators-expressions.pd",
+      "file": "07-operators-expressions.md",
       "slug": "python_en-operators_and_expressions",
       "title": "Python : Operators and Expressions"
     },
     {
-      "file": "08-control-flow.pd",
+      "file": "08-control-flow.md",
       "slug": "python_en-control_flow",
       "title": "Python : Control Flow"
     },
     {
-      "file": "09-functions.pd",
+      "file": "09-functions.md",
       "slug": "python_en-functions",
       "title": "Python : Functions"
     },
     {
-      "file": "10-modules.pd",
+      "file": "10-modules.md",
       "slug": "python_en-modules",
       "title": "Python : Modules"
     },
     {
-      "file": "11-data-structures.pd",
+      "file": "11-data-structures.md",
       "slug": "python_en-data_structures",
       "title": "Python : Data Structures"
     },
     {
-      "file": "12-problem-solving.pd",
+      "file": "12-problem-solving.md",
       "slug": "python_en-problem_solving",
       "title": "Python : Problem Solving"
     },
     {
-      "file": "13-oop.pd",
+      "file": "13-oop.md",
       "slug": "python_en-object_oriented_programming",
       "title": "Python : Object Oriented Programming"
     },
     {
-      "file": "14-io.pd",
+      "file": "14-io.md",
       "slug": "python_en-input_output",
       "title": "Python : Input Output"
     },
     {
-      "file": "15-exceptions.pd",
+      "file": "15-exceptions.md",
       "slug": "python_en-exceptions",
       "title": "Python : Exceptions"
     },
     {
-      "file": "16-standard-library.pd",
+      "file": "16-standard-library.md",
       "slug": "python_en-standard_library",
       "title": "Python : Standard Library"
     },
     {
-      "file": "17-more.pd",
+      "file": "17-more.md",
       "slug": "python_en-more",
       "title": "Python : More"
     },
     {
-      "file": "18-what-next.pd",
+      "file": "18-what-next.md",
       "slug": "python_en-what_next",
       "title": "Python : What Next"
     },
     {
-      "file": "19-appendix-floss.pd",
+      "file": "19-appendix-floss.md",
       "slug": "python_en-appendix_floss",
       "title": "Python : Appendix : FLOSS"
     },
     {
-      "file": "20-appendix-about.pd",
+      "file": "20-appendix-about.md",
       "slug": "python_en-appendix_about",
       "title": "Python : Appendix : About"
     },
     {
-      "file": "21-revision-history.pd",
+      "file": "21-revision-history.md",
       "slug": "python_en-appendix_revision_history",
       "title": "Python : Appendix : Revision History"
     },
     {
-      "file": "22-translations.pd",
+      "file": "22-translations.md",
       "slug": "python_en-translations",
       "title": "Python : Appendix : Translations"
     },
     {
-      "file": "23-translation-howto.pd",
+      "file": "23-translation-howto.md",
       "slug": "python_en-translation_howto",
       "title": "Python : Appendix : Translation Howto"
     }

requirements.txt

@@ -1,3 +1,3 @@
-Fabric == 1.6.1
-boto == 2.9.4
-beautifulsoup4 == 4.2.0
+Fabric == 1.7.0
+boto == 2.9.9
+beautifulsoup4 == 4.2.1