Wrong unsoundness?

[AntoineD]

The following reproducer code returns the __init__ method of a class:

from typing import Any
def f(cls: type) -> Any:
    return cls.__init__

but mypy considers that the __init__method was accessed from an instance:
main.py:4: error: Accessing "__init__" on an instance is unsound, since instance.__init__ could be from an incompatible subclass [misc]

Where is the problem?

[Gobot1234]

The error isn't incorrect, cls is technically an instance of type and so accessing init which isn't checked for violations of the LSP is technically unsafe

[AntoineD]

I am not sure to understand.

Also, why is the following not raising an error?

def f(cls: type) -> Any:
    return cls.__new__

[erictraut]

I don't think this should be an error. It looks to me like a mypy bug. I recommend filing a bug in the mypy issue tracker.

FWIW, this type checks without errors in pyright.

[AntoineD]

@srittau Should it be converted back to an issue?

[hauntsaninja]

Like Gobot1234 alludes to, the error exists to catch the following issue:

class A:
    def __init__(self, arg1: int) -> None: ...

class B(A):
    def __init__(self, arg1: int, arg2: str) -> None: ...

def foo(a: A):
    a.__init__(1)

foo(B(1, "one"))  # oops, TypeError

(Note that pyright has a false negative in the above cc @erictraut )

I agree that the error isn't particularly useful in your case, but your code is so reduced that I'm having a hard time understanding the use case / you may as well type ignore.

In most real life use cases that this comes up, users can get it to type check by using the class instead of the instance. That is, something like A.__init__(a, 1) in the above instead of a.__init__(1)

[AntoineD]

@hauntsaninja Thank you for the clear explanation.
I can see the potential issue, but in my case this cannot be an actual issue since only the __init__ method matters and not the result of its call (which is never performed). My use case comes from a tool to process docstrings.

[UtkarshaGupte]

Is this a bug that mypy is tracking, if yes, can anyone share the issue link? I am also seeing the same issue. Can anyone enlighten me ?

[Hnasar]

@UtkarshaGupte I don't think there's a bug, would you like to file one?