Mypy: Constructor defined after classmethod is considered to have no args in the classmethod

[alexsydell]

The type of cls within bar() in the following code is def () -> Foo unless the __init__ is moved to before the classmethod:

class Foo(object):

  @classmethod
  def bar(cls):
    # type: () -> Foo
    reveal_type(cls)
    return cls("bar")

  def __init__(self, baz):
    # type: (str) -> None
    self.baz = baz

[rzimmerman]

Probably related:

class Example:

    @classmethod
    def example(cls, arg1: float, arg2: str) -> Example:
        return cls(arg1=arg1, arg2=arg2)

    def __init__(self, arg1: float, arg2: str) -> None:
        pass

Reports:

example3.py: note: In member "example" of class "Example":
example3.py:7: error: Unexpected keyword argument "arg1" for "Example"
example3.py:7: error: Unexpected keyword argument "arg2" for "Example"

But passes checks if __init__ is moved before the classmethod.

Adding reveal_type with the __init__ method last shows:
example3.py:7: error: Revealed type is 'def () -> example3.Example'

And with __init__ first:
example3.py:11: error: Revealed type is 'def (arg1: builtins.float, arg2: builtins.str) -> example3.Example'

[ilevkivskyi]

(Raising priority to normal, since this have been reported twice after the original issue.)

[Lordnibbler]

+1 on this, thanks @ilevkivskyi

[matthewgrossman]

+1, this is also affecting my work

[mckaysalisbury]

Yeah, my behavior shows that it sometimes works in the wrong order. (see #4290)

[JelleZijlstra]

Bumping to high-pri because this is happening frequently and the workaround is non-obvious (see #2324).

[VadimPushtaev]

Hello! We encountered this bug on the very first day of mypy usage. Is there any reason it's not fixed yet? I would like to solve this issue, but it's nice to know up front if it's extremely hard to do something about it.

[JukkaL]

There probably isn't any particular reason why this hasn't been fixed -- other than there being a lot of other high-priority work as well.

This doesn't sound like it would be very hard to fix (but probably not entirely trivial either). The core team is planning to allocate a block of time focused on fixing high-priority bugs in the next couple of months, and hopefully this gets fixed then.

[VadimPushtaev]

I've spent some time trying to find a solution, and here is my attempt: https://github.com/python/mypy/pull/5501

[msullivan]

I've got a PR about to go up that fixes this as a side effect of changing the type of classmethod first arguments to be a TypeType

[ilevkivskyi]

@msullivan There is an existing PR. Please take a look at the questions that appeared there (interactions with fine-grained mode, plugins, forward references, generics, and overloads).

[ilevkivskyi]

@msullivan Forgot to add the PR number, here it is https://github.com/python/mypy/pull/5501