What does cls in new act as?

Question

recently I'm leanring about python magic method, in __new__ we always rewrite it by

class A:
    def __new__(cls):
        # ......
        return super().__new__(cls)

    def __init__(self):
        # .....

this method will return an A object then A will execute __init__.
so, I try the following script



class A:
    def __init__(self):
        print('A.__init__')
        self.a = 20


class B:
    def __new__(cls):
        print('B.__new__')
        return super().__new__(A)

    def __init__(self):
        print('B.__init__')
        self.b = 30


t = B()
print(type(t))

then I get result

B.__new__
<class '__main__.A'>

In my opinion, B.__new__ return an A object as it shows, then it should execute A.__init__ method, but in fact neither A.__init__ nor B.__init__ executed, object t is an A object but it have nothing relate to A or B, this confuse me, what does cls in __new__ method act as?

It is interesting that neither `__init__` method is executed. You're doing something that wouldn't usually be done: calling `__new__` in B to instantiate A, where the two classes are unrelated. — khelwood, Aug 31 '21 at 08:40
see https://stackoverflow.com/questions/674304/why-is-init-always-called-after-new — balderman, Aug 31 '21 at 08:41
Also see [Python (and Python C API): \_\_new\_\_ versus \_\_init\_\_](https://stackoverflow.com/questions/4859129/python-and-python-c-api-new-versus-init) — Almog-at-Nailo, Aug 31 '21 at 08:49

score 2 · Accepted Answer · answered Aug 31 '21 at 09:13

In my opinion, B.__new__ return an A object as it shows, then it should execute A.__init__ method

That assumption is incorrect. Let's look at the code that creates instances. In python this would be the type.__call__ method. The C implementation can be found here

The three interesting steps that happen are:

call type.__new__(type, ...) (here)
abort when the return value of __new__ is not a sub class of the type (here)
call __init__ on the created object (here)

Or in python code:

# t = B() is roughly equivalent to

t = B.__new__()  # which returns `A.__new__()`
if ìssubclass(type(t), B):  # False
    t.__init__()  # never called

Your implementation of B.__new__ however creates an object of type A. Because of that the issubclass check fails and no __init__ is called.

To summarize: B.__init__ is not called, because B.__new__ did not create an object of type B and A.__init__ was not called, because the class object A was not responsible for creating the instance (B was).

The end result is, that t is an instance of class A but its __init__ was not called. That's why t is not initialized.

What does cls in __new__ act as?

1 Answers1

What does cls in new act as?