python-tutorials/oop.asciidoc

[[oop]]
== 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]
--
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.

[[self]]
=== 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
[NOTE]
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`.

[[class]]
=== Classes

The simplest class possible is shown in the following example (save as `oop_simplestclass.py`).

[source,python]
--------------------------------------------------
include::programs/oop_simplestclass.py[]
--------------------------------------------------

Output:

--------------------------------------------------
include::programs/oop_simplestclass.txt[]
--------------------------------------------------

.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 <<init,more about instantiation>> 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.

[[methods]]
=== 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 `oop_method.py`).

[source,python]
--------------------------------------------------
include::programs/oop_method.py[]
--------------------------------------------------

Output:

--------------------------------------------------
include::programs/oop_method.txt[]
--------------------------------------------------

.How It Works

Here we see the `self` in action. Notice that the `say_hi` method takes no parameters but still has
the `self` in the function definition.

[[init]]
=== 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 `oop_init.py`):

[source,python]
--------------------------------------------------
include::programs/oop_init.py[]
--------------------------------------------------

Output:

--------------------------------------------------
include::programs/oop_init.txt[]
--------------------------------------------------

.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 `say_hi`
method.

[[class_obj_vars]]
=== 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 `oop_objvar.py`):

[source,python]
--------------------------------------------------
include::programs/oop_objvar.py[]
--------------------------------------------------

Output:

--------------------------------------------------
include::programs/oop_objvar.txt[]
--------------------------------------------------

.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!

Instead of `Robot.population`, we could have also used +self.__class__.population+ because every
object refers to it's class via the +self.__class__+ attribute.

The `how_many` 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 refer to a class variable, let's use `classmethod`.

We have marked the `how_many` method as a class method using a <<decorator,decorator>>.

Decorators can be imagined to be a shortcut to calling a wrapper function, so applying the
`@classmethod` decorator is same as calling:

[source,python]
--------------------------------------------------
how_many = classmethod(how_many)
--------------------------------------------------

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.say_hi.__doc__`

In the `die` method, we simply decrease the `Robot.population` count by 1.

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
[NOTE]
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 `oop_subclass.py`):

[source,python]
--------------------------------------------------
include::programs/oop_subclass.py[]
--------------------------------------------------

Output:

--------------------------------------------------
include::programs/oop_subclass.txt[]
--------------------------------------------------

.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 trailing comma is used at the end of the `print` statement in the superclass's `tell()` method
to print a line and allow the next print to continue on the same line. This is a trick to make
`print` not print a `\n` (newline) symbol at the end of the printing.

=== 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.