Constructor using `new` method

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Introduction

In Python, object creation and object initialization are intentionally separated into two distinct steps, providing a level of flexibility that many programming languages do not offer.

• Object creation is handled by the __new__ method, which is responsible for allocating memory and returning a new instance of a class.
• Object initialization is handled by the __init__ method, which configures the already-created object by assigning initial values to its attributes.

This separation allows Python to give developers fine-grained control over how and when objects are created, enabling advanced use cases such as customized object creation, immutable data handling, and design patterns like the Singleton.

What is __new__?

• __new__ is a special method responsible for creating a new object
• It is the true constructor in Python
• It is called before __init__
• It receives the class object as its first parameter, commonly named cls by convention
• It must return an instance of the class

__new__(cls)

If __new__ does not return an object, Python cannot proceed with initialization.

Sequence of Object Creation

When an object is created using:

obj = ClassName()

The execution order is:

1. __new__(cls) → creates the object
2. __init__(self) → initializes the object
3. Other instance methods can then be called

class Abc:
    def __init__(self):
        print("init called")

    def show(self):
        print("in show")

obj1 = Abc()
obj1.show()

init called
in show

obj2 = Abc.__new__(Abc)
obj2.show()

in show

class Abc:
    def __new__(cls):
        print("Constructor called")

    def __init__(self):
        print("init called")

    def show(self):
        print("in show")

obj = Abc()
obj.show()

Constructor called
AttributeError: 'NoneType' object has no attribute 'show'

class Abc:
    def __new__(cls):
        print("Constructor called")
        return super(Abc, cls).__new__(cls)

    def __init__(self):
        print("init called")

    def show(self):
        print("in show")

obj1 = Abc()
obj1.show()

Constructor called
init called
in show

obj2 = Abc.__new__(Abc)
obj2.__init__()
obj2.show()

Constructor called
init called
in show

class Singleton:
    _instance = None

    def __new__(cls):
        if cls._instance is None:
            cls._instance = super().__new__(cls)
        return cls._instance

return super().__new__(cls)

Summary

• __new__ is Python’s actual constructor and runs before __init__.
• It is responsible for creating and returning the object, while __init__ only initializes it.
• A custom __new__ must return a valid instance; otherwise, object behavior breaks.
• __init__ should be used for normal initialization, while __new__ is reserved for advanced cases.
• __new__ is essential for special object-creation logic, such as the Singleton pattern.