# 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.