Python Classes and Objects - Class 9 PDF Download

A class is a user-defined blueprint or prototype from which objects are created. Classes provide a means of bundling data and functionality together. Creating a new class creates a new type of object, allowing new instances of that type to be made. Each class instance can have attributes attached to it for maintaining its state. Class instances can also have methods (defined by their class) for modifying their state.
To understand the need for creating a class let’s consider an example, let’s say you wanted to track the number of dogs that may have different attributes like breed, age. If a list is used, the first element could be the dog’s breed while the second element could represent its age. Let’s suppose there are 100 different dogs, then how would you know which element is supposed to be which? What if you wanted to add other properties to these dogs? This lacks organization and it’s the exact need for classes.
Class creates a user-defined data structure, which holds its own data members and member functions, which can be accessed and used by creating an instance of that class. A class is like a blueprint for an object.

Some points on Python class:  

• Classes are created by keyword class.
• Attributes are the variables that belong to a class.
• Attributes are always public and can be accessed using the dot (.) operator. Eg.: Myclass.Myattribute

Class Definition Syntax:

class ClassName:

    # Statement-1

    .

    .

    .

    # Statement-N

Defining a class – 

Python3

# Python3 program to

# demonstrate defining

# a class

 

class Dog:

    pass

In the above example, the class keyword indicates that you are creating a class followed by the name of the class (Dog in this case).

Class Objects

An Object is an instance of a Class. A class is like a blueprint while an instance is a copy of the class with actual values. It’s not an idea anymore, it’s an actual dog, like a dog of breed pug who’s seven years old. You can have many dogs to create many different instances, but without the class as a guide, you would be lost, not knowing what information is required.
An object consists of : 

• State: It is represented by the attributes of an object. It also reflects the properties of an object.
• Behavior: It is represented by the methods of an object. It also reflects the response of an object to other objects.
• Identity: It gives a unique name to an object and enables one object to interact with other objects.

Declaring Objects (Also called instantiating a class)

When an object of a class is created, the class is said to be instantiated. All the instances share the attributes and the behavior of the class. But the values of those attributes, i.e. the state are unique for each object. A single class may have any number of instances.
Example:

Declaring an object – 

Python3

# Python3 program to

# demonstrate instantiating

# a class

 

 

class Dog:

     

    # A simple class

    # attribute

    attr1 = "mammal"

    attr2 = "dog"

 

    # A sample method 

    def fun(self):

        print("I'm a", self.attr1)

        print("I'm a", self.attr2)

 

# Driver code

# Object instantiation

Rodger = Dog()

 

# Accessing class attributes

# and method through objects

print(Rodger.attr1)

Rodger.fun()

Output: 

mammal

I'm a mammal

I'm a dog

In the above example, an object is created which is basically a dog named Rodger. This class only has two class attributes that tell us that Rodger is a dog and a mammal.

The self

• Class methods must have an extra first parameter in the method definition. We do not give a value for this parameter when we call the method, Python provides it.
• If we have a method that takes no arguments, then we still have to have one argument.
• This is similar to this pointer in C++ and this reference in Java.

When we 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.

__init__ method

The __init__ method is similar to constructors in C++ and Java. Constructors are used to initializing the object’s state. Like methods, a constructor also contains a collection of statements(i.e. instructions) that are executed at the time of Object creation. It runs 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.

Python3

# A Sample class with init method

class Person:

   

    # init method or constructor 

    def __init__(self, name):

        self.name = name

   

    # Sample Method 

    def say_hi(self):

        print('Hello, my name is', self.name)

   

p = Person('Nikhil')

p.say_hi()

Output: 

Hello, my name is Nikhil

Class and Instance Variables

Instance variables are for data, unique to each instance and class variables are for attributes and methods shared by all instances of the class. Instance variables are variables whose value is assigned inside a constructor or method with self whereas class variables are variables whose value is assigned in the class.
Defining instance variable using a constructor.

Python3

# Python3 program to show that the variables with a value 

# assigned in the class declaration, are class variables and

# variables inside methods and constructors are instance

# variables.

    

# Class for Dog

class Dog:

   

    # Class Variable

    animal = 'dog'            

   

    # The init method or constructor

    def __init__(self, breed, color):

     

        # Instance Variable    

        self.breed = breed

        self.color = color       

    

# Objects of Dog class

Rodger = Dog("Pug", "brown")

Buzo = Dog("Bulldog", "black")

 

print('Rodger details:')  

print('Rodger is a', Rodger.animal)

print('Breed: ', Rodger.breed)

print('Color: ', Rodger.color)

 

print('\nBuzo details:')  

print('Buzo is a', Buzo.animal)

print('Breed: ', Buzo.breed)

print('Color: ', Buzo.color)

 

# Class variables can be accessed using class

# name also

print("\nAccessing class variable using class name")

print(Dog.animal)     

Output: 

Rodger details:

Rodger is a dog

Breed:  Pug

Color:  brown

Buzo details:

Buzo is a dog

Breed:  Bulldog

Color:  black

Accessing class variable using class name

dog

Defining instance variable using the normal method.

Python3

# Python3 program to show that we can create 

# instance variables inside methods

    

# Class for Dog

class Dog:

       

    # Class Variable

    animal = 'dog'     

       

    # The init method or constructor

    def __init__(self, breed):

           

        # Instance Variable

        self.breed = breed            

   

    # Adds an instance variable 

    def setColor(self, color):

        self.color = color

       

    # Retrieves instance variable    

    def getColor(self):    

        return self.color   

   

# Driver Code

Rodger = Dog("pug")

Rodger.setColor("brown")

print(Rodger.getColor()) 

Output:  

brown