How to Use __new__ in Python: Syntax and Examples

In Python, __new__ is a special method used to create a new instance of a class before __init__ initializes it. You override __new__ when you need to control how objects are created, especially for immutable types.

Syntax

The __new__ method is a static method that takes the class as its first argument, followed by any other arguments used to create the instance. It must return a new instance of the class (usually by calling super().__new__(cls)).

• cls: The class being instantiated.
• *args, **kwargs: Arguments passed to the constructor.
• Returns a new instance of cls.

python

def __new__(cls, *args, **kwargs):
    instance = super().__new__(cls)
    # custom creation logic here
    return instance

Example

This example shows how to override __new__ to print a message when creating an object and then initialize it with __init__.

python

class MyClass:
    def __new__(cls, *args, **kwargs):
        print("Creating instance...")
        instance = super().__new__(cls)
        return instance

    def __init__(self, value):
        print("Initializing instance...")
        self.value = value

obj = MyClass(10)
print(obj.value)

Output

Creating instance...
Initializing instance...
10

Common Pitfalls

Common mistakes when using __new__ include:

• Not returning the new instance from __new__, which causes __init__ not to run.
• Using __new__ unnecessarily for mutable objects where __init__ is enough.
• Forgetting that __new__ is a static method and must accept the class as the first argument.

python

class Wrong:
    def __new__(cls):
        print("Creating instance but not returning it")

    def __init__(self):
        print("Initializing")

# This will raise an error because __new__ returns None
try:
    obj = Wrong()
except TypeError as e:
    print(e)


class Right:
    def __new__(cls):
        print("Creating instance and returning it")
        return super().__new__(cls)

    def __init__(self):
        print("Initializing")

obj = Right()

Output

Creating instance but not returning it
__init__() missing 1 required positional argument: 'self'
Creating instance and returning it
Initializing

Quick Reference

Use __new__ to control object creation, especially for immutable types like tuples or strings. Always return the new instance. Use __init__ for setting up instance attributes after creation.

Method	Purpose	When to Use
__new__(cls, *args, **kwargs)	Create and return a new instance	Control instance creation, immutable objects
__init__(self, *args, **kwargs)	Initialize instance attributes	Set up after creation

Key Takeaways

Override __new__ to customize how a new object is created before initialization.

__new__ must return the new instance; otherwise, __init__ will not run.

Use __new__ mainly for immutable types or special creation logic.

For most cases, __init__ is enough to initialize objects after creation.

Remember __new__ is a static method and receives the class as the first argument.