Difference between init and new in Python

__init__ and __new__ are dunder methods invoked during the creation of an object instance.

What are the differences?

1. __new__ is used to control the creation of a new instance
2. __init__ is used to control the initialization of an instance

__new__ is the first step of an instance creation and is responsible for returning a new instance of the class.

In contrast, the __init__ function initializes the instance after its creation.

Code example

In the code snippet below, __new__ contains the cls class argument while __init__ contains the self reference to the instance.

 1class Example:
 2    def __new__(cls, *args, **kwargs):
 3        print("__new__")
 4        return super().__new__(cls, *args, **kwargs)
 5
 6    def __init__(self):
 7        print("__init__")
 8
 9
10e = Example()

Output:

1__new__
2__init__

Increment an invoked_count variable each time __new__ is called:

 1class Example:
 2    invoked_count = 0
 3
 4    def __new__(cls, *args, **kwargs):
 5        Example.invoked_count += 1
 6        return super().__new__(cls, *args, **kwargs)
 7
 8    def __init__(self):
 9        print(Example.invoked_count)
10
11
12e1 = Example()
13e2 = Example()

Output

11
22

Use cases

__init__ is used as a class constructor for initializing new instances.

A common use case for __new__ is implementing the singleton pattern.