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Optional generic not accepted with --strict-optional #2678
Description
When running with --strict-optional, mypy complains about the last statement here. However, this seems like it should work, both because it does work when --strict-optional is left out, and because the revealed type looks like it should match.
from typing import Optional, Type, TypeVar
class BaseThing: pass
class ChildThing(BaseThing): pass
ThingClass = TypeVar("ThingClass", bound=BaseThing)
def get_thing(thing_class: Type[ThingClass]) -> Optional[ThingClass]:
""" Get the thing with the given class. Stub implementation. """
return None
thing = None # type: Optional[ChildThing]
# Fails, complaining about `Type argument 1 of "get_thing" has incompatible value "Optional[ChildThing]"`
thing = get_thing(ChildThing)
# Revealed type is `Union[scratch_1.ChildThing*, builtins.None]`, which seems like it should
# match Optional[ChildThing].
reveal_type(get_thing(ChildThing))
Activity
gvanrossum commentedon Jan 11, 2017
Hm, I wonder if the problem is due to a bad interaction between the ThingClass TypeVar and the Optional from the context? I think it ought to infer that if the context is Optional[ChildThing] and the return type (to be matched to that context) is Optional[ThingClass], then it must solve this to ThingClass==ChildThing, but it is then not correctly concluding that the argument type must be Type[ChildThing].
ddfisher commentedon Jan 11, 2017
This problem isn't Optional-specific -- it'll happen with any Union. For example:
We've known for a while that TypeVars and Unions don't play well together. There's been a bit of spot-patching so far, but I think we're at the point where a larger careful rethinking makes sense.
Improve usage of outer context for inference (#5699)
Improve usage of outer context for inference (python#5699)