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

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

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'

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

@ilevkivskyi

+1 on this, thanks @ilevkivskyi

+1, this is also affecting my work

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

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

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.

Fixes python#1727: constructor after classmethod counts

I've spent some time trying to find a solution, and here is my attempt: #5501

Fixes python#1727: constructor after classmethod counts

python#1727: deps.test for classmethod

python#1727: fine-grained.test for classmethod

python#1727: more unit tests

python#1727: test classmethod in nested class

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

@msullivan

@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).

@msullivan

@msullivan Forgot to add the PR number, here it is #5501

Make classmethod's first argument be Type[...]

Make classmethod's first argument be Type[...] (#5646)

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Constructor defined after classmethod is considered to have no args in the classmethod #1727

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rzimmerman commented on Sep 5, 2016

ilevkivskyi commented on Aug 7, 2017

Lordnibbler commented on Aug 7, 2017

matthewgrossman commented on Aug 9, 2017

mckaysalisbury commented on Nov 30, 2017

JelleZijlstra commented on Dec 5, 2017

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JukkaL commented on Aug 16, 2018

VadimPushtaev commented on Aug 18, 2018

msullivan commented on Sep 19, 2018

ilevkivskyi commented on Sep 19, 2018

ilevkivskyi commented on Sep 19, 2018

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