Generic Self classmethod fails for generic types

Repro:

from typing import Generic, TypeVar, Type

_T = TypeVar('_T')
SelfType = TypeVar('SelfType', bound='Friend')

class Friend(Generic[_T]):
    @classmethod
    def make(cls: Type[SelfType]) -> SelfType:  # E: Unsupported type Type["SelfType"]
        return cls()

I suspect the issue is that the type should be:

def make(cls: Type[SelfType[T]]) -> SelfType[T]:
        return cls()

But there is no way to express that that I can tell.

Using an instance method behaves as expected:

from typing import Generic, TypeVar, Type

_T = TypeVar('_T')
SelfType = TypeVar('SelfType', bound='Friend')

class Friend(Generic[_T]):
    other = None  # type: Friend

    def make(self: SelfType) -> SelfType:
        return self

I think this is a duplicate of #3363 and #2354
I think we could leave only one of these three issues open and raise its priority.

@ilevkivskyi

@ilevkivskyi I'm fine with that. AFAICT #2354 is effectively fixed, the code example below works great:

from typing import Generic, TypeVar, Type

_T = TypeVar('_T')
SelfType = TypeVar('SelfType', bound='Friend')

class Friend(Generic[_T]):
    other = None  # type: Friend

    def make(self: SelfType) -> SelfType:
        return self

class SuperFriend(Generic[_T], Friend[_T]):
    pass


reveal_type(SuperFriend[int]().make())  # E: Revealed type is 'test_generic_self.SuperFriend[builtins.int]'

I'd be fine if classmethods had the same behavior. In fact, @elazarg 's change #3633 mostly fixes this issue. The types (as opposed to <nothing>) are correct but declaring the function make still throws an error:

from typing import Generic, TypeVar, Type

_T = TypeVar('_T')
SelfType = TypeVar('SelfType', bound='Friend')

class Friend(Generic[_T]):
    other = None  # type: Friend

    @classmethod
    def make(cls: Type[SelfType]) -> SelfType:  # E: Unsupported type Type["SelfType"]
        return cls()

class SuperFriend(Generic[_T], Friend[_T]):
    pass


reveal_type(SuperFriend[int]().make())  # E:  Revealed type is 'test_generic_self.SuperFriend*[builtins.int*]'

I think the Unsupported type Type["SelfType"] error is a separate issue. It happens even for staticmethod, for which there's no specific check for selftype.

@elazarg

@elazarg got it. If you can land #2354 I can debug the Type["SelfType"] issue.

Yeah, it gives the same error when `Type[SelfType]` is used outside the `Friend` class. It's purely because of the use of Type[] with a type variable that is bound by a generic class.

I've been sent here by stackoverflow. Issue is still relevant.

Allow calling Type[T] where T has generic bound (#5309)

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Generic Self classmethod fails for generic types #3631

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Generic Self classmethod fails for generic types #3631

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ilevkivskyi commented on Jun 29, 2017

rowillia commented on Jun 29, 2017

elazarg commented on Jun 30, 2017

rowillia commented on Jun 30, 2017

gvanrossum commented on Jun 30, 2017

bharel commented on Jun 29, 2018

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