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bluetech

opened on Mar 1, 2020

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OrderedDict inherits fromkeys() from its parent Dict. dict.fromkeys has hardcoded return type dict:

typeshed/stdlib/2and3/builtins.pyi

Lines 999 to 1004 in 9500296

	@staticmethod
	@overload
	def fromkeys(__iterable: Iterable[_T]) -> Dict[_T, Any]: ... # TODO: Actually a class method (mypy/issues#328)
	@staticmethod
	@overload
	def fromkeys(__iterable: Iterable[_T], __value: _S) -> Dict[_T, _S]: ...

Easy way to fix this is to explicitly override to return OrderedDict, but this doesn't take care of the same problem in other dict subclasses.

I tried to fix it using generic self. For the first (short) form of fromkeys(), I think it should work:

class dict(MutableMapping[_KT, _VT], Generic[_KT, _VT]):
    _FK1 = TypeVar('_FK1', bound='dict[_KT, None]')

    @classmethod
    def fromkeys(cls: Type[_FK1], __iterable: Iterable[_T]) -> _FK1: ...

This uses the method described in the docs.

But the second (long) form has a complication - the value type of the dict depends on a generic argument to fromkeys. The following doesn't work:

class dict(MutableMapping[_KT, _VT], Generic[_KT, _VT]):
    _FK2 = TypeVar('_FK2', bound='dict[_KT, _S]')

    @classmethod
    def fromkeys(cls: Type[_FK2], __iterable: Iterable[_T], __value: _S) -> _FK2: ...

Understandably fails with Type variable "builtins._S" is unbound in the _FK2 = ... line.

I wonder if there is a way to make this work?

Reacted by Andrew

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1. JelleZijlstra commented on Mar 2, 2020

   

   JelleZijlstra

   on Mar 2, 2020

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   Wow, I didn't know of that trick (defining a typevar within the class) to make the key type work. I can't think of a way to make the value type work correctly either.

   I think we should just add a stub for OrderedDict.fromkeys and not worry about other possible dict subclasses unless user reports come in that indicate that it's a common problem.
2. bluetech commented on Mar 5, 2020

   

   bluetech

   on Mar 5, 2020

   ContributorAuthor

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   I think we should just add a stub for OrderedDict.fromkeys and not worry about other possible dict subclasses unless user reports come in that indicate that it's a common problem.

   Sure. I'll do that after #3799 is settled to avoid a (semantic) conflict.
3. ssbarnea commented on Jul 25, 2020

   

   ssbarnea

   on Jul 25, 2020

   SponsorContributor

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   Is there a temporary workaround that we can use until this is sorted?
4. AlexWaygood

   added a commit that references this issue on Dec 3, 2021

   Add `OrderedDict.fromkeys`

   This issue will close once commit c00a660 is merged into the 'main' branch.

   c00a660
5. Akuli

   closed this as completedin #6485on Dec 3, 2021

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