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__init__ in Python

  • Difficulty Level : Easy
  • Last Updated : 26 Nov, 2019

Prerequisites – Python Class, Objects, Self

Whenever object oriented programming is done in Python, we mostly come across __init__ method which we usually don’t fully understand. This article explains the main concept of __init__ but before understanding the __init__ some prerequisites are required.

__init__

The __init__ method is similar to constructors in C++ and Java. Constructors are used to initialize the object’s state. The task of constructors is to initialize(assign values) to the data members of the class when an object of class is created. Like methods, a constructor also contains collection of statements(i.e. instructions) that are executed at time of Object creation. It 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.

Example:




# 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

Understanding the code

In the above example, a person name Nikhil is created. While creating a person, “Nikhil” is passed as an argument, this argument will be passed to the __init__ method to initialize the object. The keyword self represents the instance of a class and binds the attributes with the given arguments. Similarly, many objects of Person class can be created by passing different names as arguments.

Example:




# 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)  
  
# Creating different objects     
p1 = Person('Nikhil')  
p2 = Person('Abhinav')
p3 = Person('Anshul')
  
p1.say_hi()  
p2.say_hi()
p3.say_hi()

Output:

Hello, my name is Nikhil
Hello, my name is Abhinav
Hello, my name is Anshul

__init__ with inheritance

Inheritance is the capability of one class to derive or inherit the properties from some other class. Let’s consider the below example to see how __init__ works in inheritance.




# Python program to
# demonstrate init with
# inheritance
  
class A(object):
    def __init__(self, something):
        print("A init called")
        self.something = something
          
  
class B(A):
    def __init__(self, something):
        # Calling init of parent class
        A.__init__(self, something)
        print("B init called")
        self.something = something
          
obj = B("Something")

Output:

A init called
B init called

So, the parent class constructor is called first. But in Python, it is not compulsory that parent class constructor will always be called first. The order in which the __init__ method is called for a parent or a child class can be modified. This can simply be done by calling the parent class constructor after the body of child class constructor.

Example:




# Python program to
# demonstrate init with
# inheritance
  
class A(object):
    def __init__(self, something):
        print("A init called")
        self.something = something
          
  
class B(A):
    def __init__(self, something):
        print("B init called")
        self.something = something
        # Calling init of parent class
        A.__init__(self, something)
          
obj = B("Something")

Output:

B init called
A init called

Note: To know more about inheritance click here.

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