What is the difference between the __init__ and __new__ methods in Python?

__new__ — a static method (implicitly receives the class cls as the first argument, not the instance self) responsible for creating an instance of an object. It is called before __init__. If __new__ returns an instance of the class it was called on (or its subclass), then the __init__ method is called to initialize this instance. If __new__ returns an instance of another class, __init__ is not called.

__init__ — an instance method (receives the instance self as the first argument), responsible for initializing (setting up) an already created object instance.

Use of __new__ is usually required in the following cases:

• Changing behavior when creating instances of immutable types (strings, numbers, tuples).
• Implementing patterns such as Singleton.
• Inheriting from immutable built-in types.

Example of using __new__ to implement Singleton:

class Singleton:
    _instance = None

    def __new__(cls, *args, **kwargs):
        if cls._instance is None:
            cls._instance = super().__new__(cls) # Call __new__ of the base class to create an instance
        return cls._instance

    def __init__(self, value):
        # __init__ will be called every time if __new__ returns the same instance
        self.value = value

# Creating instances
s1 = Singleton(1)
s2 = Singleton(2)

# Check that these are the same instance
# print(s1 is s2) # Output: True
# print(s1.value) # Output: 2
# print(s2.value) # Output: 2

Example of using __init__:

class MyClass:
    def __init__(self, name):
        self.name = name # Initialize the attribute

    def greet(self):
        return f"Hello, {self.name}!"

# Creating an instance and calling a method
# obj = MyClass("World")
# print(obj.greet()) # Output: Hello, World!

Comparison:

Method	Purpose	Method Type	First Argument	When Called	Typical Use Cases
__new__	Creating an instance	Static	cls (class)	Before __init__	Overriding creation of immutable type instances, Singleton, inheriting from immutable built-in types
__init__	Initializing an instance	Instance	self (instance)	After __new__ (if __new__ returned an instance of the class)	Setting up the created instance, assigning attribute values