Why is Python's `__init__()` Always Called After `__new__()`?

Understanding the Sequence of __init__() and __new__()

In Python, the __new__() and __init__() methods play crucial roles in the instance creation process. However, many developers wonder why __init__() is always called after __new__().

Purpose of __new__()

__new__() is responsible for controlling the creation of a new instance. It initializes the instance's allocation and returns the instance itself. Essentially, __new__() decides whether to create a new instance or reuse an existing one.

Purpose of __init__()

__init__() is used for object initialization. It sets the instance's attributes and runs any necessary setup code. __init__() doesn't return anything, as its primary purpose is to initialize the object's state.

Sequence in Instance Creation

The sequence in instance creation is always:

1. __new__() is called first to allocate and potentially return an existing instance.
2. If __new__() returns a new instance, __init__() is then called to initialize the instance.

Example

The following example illustrates how __new__() controls instance creation:

class A(object):
    _dict = dict()

    def __new__(cls):
        if 'key' in A._dict:
            print("EXISTS")
            return A._dict['key']
        else:
            print("NEW")
            return super(A, cls).__new__(cls)

    def __init__(self):
        print("INIT")
        A._dict['key'] = self
        print("")

a1 = A()
a2 = A()
a3 = A()

Output:

NEW
INIT

EXISTS
INIT

EXISTS
INIT

As you can see, __new__() first checks if an instance already exists in _dict. If so, it returns that existing instance. Otherwise, it creates a new instance and returns it. __init__() is then called to initialize each instance.

Alternatives to Using __new__()

In most cases, using __new__() to implement your own flyweight design pattern is not recommended. Instead, consider using a factory class or an object pool to manage the creation and reuse of instances.

The above is the detailed content of Why is Python's `__init__()` Always Called After `__new__()`?. For more information, please follow other related articles on the PHP Chinese website!