Python en:Object Oriented Programming

Introduction
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</tt> in Python is equivalent to the this</tt> pointer in C++ and the this</tt> reference in Java and C#.

You must be wondering how Python gives the value for self</tt> 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</tt> and an instance of this class called myobject</tt>. When you call a method of this object as  myobject.method(arg1, arg2)</tt>, this is automatically converted by Python into MyClass.method(myobject, arg1, arg2)</tt> - this is all the special self</tt> 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</tt>.

Classes
The simplest class possible is shown in the following example.

Output:

$ python simplestclass.py   <__main__.Person object at 0x019F85F0>

How It Works:

We create a new class using the class</tt> 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</tt> 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 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</tt> class in the __main__</tt> 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</tt> variable. We will now see an example.

Output:

$ python method.py   Hello, how are you?

How It Works:

Here we see the self</tt> in action. Notice that the <tt>sayHi</tt> method takes no parameters but still has the <tt>self</tt> 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 <tt>__init__</tt> method now.

The <tt>__init__</tt> 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:

Output:

$ python class_init.py   Hello, my name is Swaroop

How It Works:

Here, we define the <tt>__init__</tt> method as taking a parameter <tt>name</tt> (along with the usual <tt>self</tt>). Here, we just create a new field also called <tt>name</tt>. Notice these are two different variables even though they are both called 'name'. The dotted notation allows us to differentiate between them.

Most importantly, notice that we do not explicitly call the <tt>__init__</tt> 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 <tt>self.name</tt> field in our methods which is demonstrated in the <tt>sayHi</tt> 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:

Output:

(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, <tt>population</tt> belongs to the <tt>Robot</tt> class and hence is a class variable. The <tt>name</tt> variable belongs to the object (it is assigned using <tt>self</tt>) and hence is an object variable.

Thus, we refer to the <tt>population</tt> class variable as <tt>Robot.population</tt> and not as <tt>self.population</tt>. We refer to the object variable <tt>name</tt> using <tt>self.name</tt> 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 <tt>howMany</tt> is actually a method that belongs to the class and not to the object. This means we can define it as either a <tt>classmethod</tt> or a <tt>staticmethod</tt> depending on whether we need to know which class we are part of. Since we don't need such information, we will go for <tt>staticmethod</tt>.

We could have also achieved the same using decorators:

Decorators can be imagined to be a shortcut to calling an explicit statement, as we have seen in this example.

Observe that the <tt>__init__</tt> method is used to initialize the <tt>Robot</tt> instance with a name. In this method, we increase the <tt>population</tt> count by 1 since we have one more robot being added. Also observe that the values of <tt>self.name</tt> 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 <tt>self</tt> 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 <tt>Robot.__doc__</tt> and the method docstring as <tt>Robot.sayHi.__doc__</tt>

Just like the <tt>__init__</tt> method, there is another special method <tt>__del__</tt> 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 <tt>Robot.population</tt> count by 1.

The <tt>__del__</tt> 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 <tt>del</tt> statement which is what we have done here.


 * Note for C++/Java/C# Programmers
 * All class members (including the data members) are public and all the methods are virtual in Python.
 * One exception: If you use data members with names using the double underscore prefix such as <tt>__privatevar</tt>, 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).

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 <tt>SchoolMember</tt> 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 <tt>SchoolMember</tt>, 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 <tt>SchoolMember</tt> 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 <tt>SchoolMember</tt> class in this situation is known as the base class or the superclass. The <tt>Teacher</tt> and <tt>Student</tt> classes are called the derived classes or subclasses.

We will now see this example as a program.

Output:

$ python 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 <tt>__init__</tt> method of the base class is explicitly called using the <tt>self</tt> 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 <tt>self</tt> variable along with any arguments.

Notice that we can treat instances of <tt>Teacher</tt> or <tt>Student</tt> as just instances of the <tt>SchoolMember</tt> when we use the <tt>tell</tt> method of the <tt>SchoolMember</tt> class.

Also, observe that the <tt>tell</tt> method of the subtype is called and not the <tt>tell</tt> method of the <tt>SchoolMember</tt> 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.

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 * TODO
 * It is too sudden to introduce this concept here.

Metaclasses
There is much more to the vast topic of object-oriented programming, but we'll touch upon a few topics lightly here so that we become aware that such concepts exist.

Just like we use classes to create objects, we can use metaclasses to create classes. Metaclasses are used to modify or introduce new behavior to classes.

Let us take an example. Let's say we want to make sure that we always create instances of only subclasses of the <tt>SchoolMember</tt> class and that we do not create instances of the <tt>SchoolMember</tt> class itself.

We can achieve this by using a concept called "abstract base classes". All that it means is that it is an abstract class which means it's a concept, and not meant to be used as a real class.

We can declare that our class is an abstract base class by using a built-in metaclass called <tt>ABCMeta</tt>.

Output:

$ python test.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:

We can declare the 'tell' method of the 'SchoolMember' class as an abstract method, and hence automatically we are not allowed to create an instance of the SchoolMember class.

However, we can process instances of <tt>Teacher</tt> and <tt>Student</tt> as if they are <tt>SchoolMember</tt> instances because they are subclasses.

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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.

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