8000 Generic Self classmethod fails for generic types · Issue #3631 · python/mypy · GitHub
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rowillia opened this issue Jun 29, 2017 · 6 comments
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Generic Self classmethod fails for generic types #3631

rowillia opened this issue Jun 29, 2017 · 6 comments

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@rowillia
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rowillia commented Jun 29, 2017 •
edited by gvanrossum
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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
@ilevkivskyi
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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.

@rowillia
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rowillia commented Jun 29, 2017 •
edited
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@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*]'

@elazarg
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elazarg commented Jun 30, 2017

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.

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

@gvanrossum
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gvanrossum commented Jun 30, 2017 via email

@JelleZijlstra JelleZijlstra mentioned this issue Nov 4, 2017
Merged
@bharel
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bharel commented Jun 29, 2018

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

@elazarg elazarg mentioned this issue Jul 3, 2018
Merged
ilevkivskyi pushed a commit that referenced this issue Aug 2, 2018
@elazarg @ilevkivskyi
Allow calling Type[T] where T has generic bound (#5309)
573be3e 
Fix #3631 : looks like it was an explicit decision, made before we had self types.

The code modified in this PR was introduced at #1569. There are no tests for the current behavior, and it seems like we actually want to allow it.
4AED
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5 participants
@bharel @rowillia @gvanrossum @elazarg @ilevkivskyi
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