Classes and Objects in Python

Classes and objects are backbones of object-oriented programming in any programming language. In object-oriented programming, any entity that is capable of performing any function and has some attributes is implemented as a class. In this article, you will study what classes and objects are, the differences between the two, and how to implement them in Python.

Classes

A class serves as a blueprint for creating objects. A class is similar to the map of a house. With the help of a map, you can find the number of bedrooms, bathrooms, dining rooms, and other parts of a house. The same is the case with a class. With class, you define the types of attributes and methods that the object of a class will contain. You can build multiple houses using one map. In the same way, you can create multiple objects using one class.

Syntax of a Python Class

class ClassName:
    '''This is the body of the classs'''
    pass

Let’s now create a meaningful class. Suppose, you are developing a racing car game and you have to develop a class for the entity Vehicle. You can do so as follows:

class Vehicle:
    
    ## Instance Methods
    def show_details(self):
        
        self.vehicle_name = "Honda"
        self.vehicle_price = 10000
        
        print(self.vehicle_name)
        print(self.vehicle_price)
    

The script above creates a class named Vehicle. The Vehicle class contains one member method show_details() and two member variables: vehicle_name and vehicle_price. It is pertinent to mention that the instance variables are initialized inside an instance method and the first parameter to the instance method by default is self. The show_details() method is an instance method that shows the values of both the member variables.

Objects

Objects are instances of a class, hence, the process of creating objects is called instantiation. Let’s create a simple object of the Vehicle class.

my_car = Vehicle()
my_car.show_details()

In the script above, we create an object of the Vehicle class called my_car. To access the object’s method, you have to write the name of the object followed by a dot operator and the name of the function. In the script above we call the my_car.show_details() function to display the vehicle’s details. Here is the output:

Honda
10000

Similarly, you can access the object’s member like this:

print(my_car.vehicle_name)
print(my_car.vehicle_price)

Class Constructor

A constructor is a special method that is called whenever you create an object of a class. You need to define __init__() method in order to create a class constructor. The following script adds a constructor to our Vehicle class. The variables vehicle_name and vehicle_price are initialized within the constructor.

class Vehicle:
    
    def __init__(self, name, price):
        self.vehicle_name = name
        self.vehicle_price = price
        
    def show_details(self):
        
        print(self.vehicle_name)
        print(self.vehicle_price)

In the above script, the constructor has three parameters: self, name and price. The first parameter i.e. self refers to the object that is being instantiated, you do not have to pass the value for the parameter self. However, while creating the object of the Vehicle class, you have to pass the values for the name and price parameters . These parameter values initialize the vehicle_name and vehicle_price variables respectively.

Let’s create two objects of the Vehicle class using the class constructor:

my_car2 = Vehicle("Ford", 25000)
my_car2.show_details()

my_car3 = Vehicle("Toyota", 10000)
my_car3.show_details()

Output:

Ford
25000
Toyota
10000

Instance Members vs. Class Members

A class can have two types of members (variables and methods): instance members and class members. Instance members as the name suggests are those variables or methods that belong to an individual object and are not shared among objects of the same class. Class members, on the other hand, consist of variables and methods that are shared by all the instances of a class. The following example explains this concept.

class Vehicle:
    
    ## class variable
    count = 0
    
    ## constructor
    def __init__(self, name, price):
        self.vehicle_name = name
        self.vehicle_price = price
        Vehicle.count += 1
    
    ## instance method
    def show_details(self):
        
        print(self.vehicle_name)
        print(self.vehicle_price)
        
    ## class method    
    def show_vehicle_count():
        print(Vehicle.count)

In the above script, we define a Vehicle class that contains one class variable count and two instance variables vehicle_name and vehicle price. As seen in the script above, to create instance variables, you need to prefix the keyword self before the variable names. Also, you have to initialize instance variables inside an instance method. On the other hand, class variables are defined outside any method or constructor and you can access them via the class name.

The Vehicle class also contains an instance method show_details() and a class method show_vehicle_count(). The difference between a class method and instance method is the self parameter which is the first parameter in the case of an instance method. On the contrary, a class method doesn’t contain any self parameter.

In the Vehicle class, the class constructor initializes the vehicle_name and vehicle_price variables, while it increments the class variable count. Let’s create objects of the Vehicle class and see the difference between class and instance variables:

my_car4 = Vehicle("BWM", 300000) 
my_car4.show_details() ## calling instance method
Vehicle.show_vehicle_count() ## calling class method

In the script above, we create an object of the Vehicle class and then call the show_details() and show_vehicle_count methods. You can see that we call the instance method show_details() via the object name and the class method show_vehicle_count() via the class name. Since the initial value of the count variable is 0, creating an object of the Vehicle class will increment it to 1. Here is the output:

BWM
300000
1

Now when you create another object of the Vehicle class, the value of the count variable will be incremented to 2 since all the instances share this variable. Therefore, you will see 2 in the output as shown below:

my_car5 = Vehicle("Renault", 26000) 
my_car5.show_details() ## calling instance method
Vehicle.show_vehicle_count() ## calling class method

Output:

Renault
26000
2