__init__ in Python

Prerequisites - Python Class and Objects, Self

__init__ method in Python is used to initialize objects of a class. It is also called a constructor. It is like a default constructor in C++ and Java. Constructors are used to initialize the object’s state.

The task of constructors is to initialize (assign values) to data members of the class when an object of the class is created. Like methods, a constructor also contains a collection of statements(i.e. instructions) that are executed at the time of Object creation. It is run as soon as an object of a class is instantiated.

# 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 named 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 the Person class can be created by passing different names as arguments.

__init__ with Parameter

# 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__ Method 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 the 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 the 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

Read: Inheritance in Python

Related Article

• Is __init__() a private method in Python?
• What is the difference between __init__ and __call__?
• __new__ in Python