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OrderedDict.fromkeys() returns Dict instead of OrderedDict #3800
Description
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
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?
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JelleZijlstra commentedon Mar 2, 2020
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.fromkeysand not worry about other possible dict subclasses unless user reports come in that indicate that it's a common problem.bluetech commentedon Mar 5, 2020
Sure. I'll do that after #3799 is settled to avoid a (semantic) conflict.
ssbarnea commentedon Jul 25, 2020
Is there a temporary workaround that we can use until this is sorted?
Add `OrderedDict.fromkeys`