Python - Classes and Objects

Python supports Object Oriented Programming (OOPs).

In OOPs, the basic building blocks are:

• Class
• Object

This page starts by explaining the concept of a Class.

• In Python, everything is an object.
• To create objects, we require some Model, Plan, or Blueprint.
• This Model, Plan, or Blueprint is called a Class.
• We can write a class to represent the properties (attributes) and actions (behaviour) of an object.
• Properties are represented by Variables.
• Actions are represented by Methods.
• Hence, a class contains both variables and methods.

                     Class
                       │
              ┌────────┴────────┐
              │                 │
          Variables         Methods
         (Properties)     (Behaviour)
        

A class contains two important parts.

Part	Represents
Variables	Properties (Attributes)
Methods	Actions (Behaviour)

We can define a class by using the class keyword.

1. class

The class keyword is used to define a class.

2. className

This is the name of the class.

By convention, class names begin with a capital letter.

A documentation string represents the description of the class.

Within a Python class, the documentation string is optional.

Python provides two ways to access the documentation string.

Method 1

Use the __doc__ attribute of the class.

No output captured.

Use the help() built-in function.

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• class Student: defines a class named Student.
• ''' This is student class with required data ''' is the documentation string of the class.
• print(Student.__doc__) prints the documentation string.
• help(Student) displays the help information of the class.

The following example demonstrates a simple Python class.

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The class is named Student. It contains:

• A documentation string.
• A constructor (__init__()).
• Three variables: name, age, and marks.
• One method: talk().

The talk() method prints the student's information.

Within a Python class, we can represent data by using variables.

There are three types of variables:

1. Instance Variables (Object Level Variables)
2. Static Variables (Class Level Variables)
3. Local Variables (Method Level Variables)

Variable Type	Description
Instance Variable	Belongs to an object. Every object has its own separate copy.
Static (Class) Variable	Shared among all objects of the class.
Local Variable	Declared inside a method and accessible only within that method.

Within a Python class, we can represent operations by using methods.

There are three types of methods:

1. Instance Methods
2. Class Methods
3. Static Methods

Method Type	Description
Instance Methods	Operate on object data.
Class Methods	Operate on class data.
Static Methods	General utility methods.

Physical existence of a class is nothing but an object.

We can create any number of objects for a class.

A class is only a Blueprint or Model.

When we create the real implementation of that blueprint, it is called an Object.

Class (Blueprint)
        │
        ▼
Create Object
        │
        ▼
Real Physical Existence
        

The variable which can be used to refer to an object is called a Reference Variable.

By using the reference variable, we can access the properties and methods of the object.

              Student Object
                    ▲
                    │
                    │
                    s
        

Here, s is the reference variable and it refers to the Student object.

Member	Example
Properties (Variables)	s.name
Methods	s.talk()

• class Student: defines a class named Student.
• The constructor receives name, rollno, and marks, and stores them inside the object using self.
• s = Student("Durga", 101, 90) creates a Student object and stores its reference in s.
• s.talk() calls the talk() method using the reference variable.

        Class
          │
          ▼
     Student()
          │
          ▼
        Object
          ▲
          │
          │
          s
 (Reference Variable)
        

The class defines the blueprint, Student() creates the object, and the reference variable s is used to access the object.