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Callable and Tuple should be subclassable. #299
Description
One can subclass collections.abc.Mapping as well as typing.Mapping[str, int]. However, it is currently prohibited to subclass typing.Callable and typing.Tuple:
class C(collections.abc.Callable): ... # works
class C(typing.Callable): ... # fails
class C(typing.List[T]): ... # works
class C(typing.Tuple[T, ...]): ... # failsI think that both Callable and Tuple should be subclassable (at least in the simple form shown above).
@gvanrossum If you agree, then you can assign this issue to me.
Activity
gvanrossum commentedon Oct 16, 2016
I'm not entirely against this, but IIRC this is intentional, since these
are not plain generic classes. Doing the right thing in the type checker
might be difficult. I think Callable should basically be treated as a duck
type (call) and if you want to subclass Tuple you should subclass tuple
instead. Probably. Maybe @JukkaL has more thoughts?
JukkaL commentedon Oct 16, 2016
Early prototypes of
typingactually supported subclassingTuple[...]. I think that it's as reasonable as subclassingList[...], which is already supported. Subclassingtupleis not really a good alternative because then you can't specify the item types or the number of items, and instead it falls back toTuple[Any, ...].I think that
Callableshould use structural subtyping -- if__call__is defined, it's a callable. Explicitly subclassing it seems kind of reasonable as well, for consistency with thecollections.abc.CallableABC, but it would be less useful than subclassingTuple[...].Refactor Union, Tuple, and Callable (#308)