filtering with isinstance doesn't narrow down types

[YtvwlD]

I had about this code (this is a small example):

from typing import List, Union

class TypeA: pass
class TypeB: pass

Types = Union[TypeA, TypeB]
t: List[Types] = [TypeA(), TypeB()]
a: List[TypeA] = list(filter(lambda x: isinstance(x, TypeA), t))

And I got the following message (both with my distribution's mypy 0.560 and with the current master):

error: Argument 2 to "filter" has incompatible type "List[Union[TypeA, TypeB]]"; expected "Iterable[TypeA]"

When I remove the type after a, it treats a as List[Types].

I get that this might be difficult, but it would be nice if it worked.

This could be related to #476, I'm not sure.

[WillAyd]

I think this would be another example if useful:

from pathlib import Path
from typing import Union

def maybe_lowercase(obj: Union[str, Path]) -> Union[str, Path]:
    if isinstance(obj, str):
        return obj.lower()

    return obj

def foo(obj: Union[str, Path]) -> str:
    if isinstance(obj, Path):
        return str(obj)

    return maybe_lowercase(obj)

In foo the last return cannot be a Path given the preceding isinstance check, but it doesn't seem to narrow the types and complains because maybe_lowercase is typed to return a Path in it's union:

error: Incompatible return value type (got "Union[str, Path]", expected "str")

[JelleZijlstra]

@WillAyd your example should use a TypeVar with value restriction to str and Path for maybe_lowercase not a union. The signature you use does not mean that maybe_lowercase will return a str when given a str.

[ilevkivskyi]

I get that this might be difficult, but it would be nice if it worked.

Yes, this is too tricky. There is a plan to support custom type guards, you can probably use that instead. I am leaving this open as a remainder about the use case.

[AlexWaygood]

TypeGuard has now been implemented in mypy, and the typeshed stub for filter has been updated to include an overload for TypeGuard functions. That means if I modify the original repro to this:

from typing import List, Union, TypeGuard

class TypeA: pass
class TypeB: pass

def is_TypeA(x: object) -> TypeGuard[TypeA]:
    return isinstance(x, TypeA)

Types = Union[TypeA, TypeB]
t: List[Types] = [TypeA(), TypeB()]
a: List[TypeA] = list(filter(is_TypeA, t))

and then (inside my local clone of typeshed) run mypy test_script.py --custom-typeshed-dir ., I get no errors. Hooray!

mypy's vendored copy of typeshed has yet to include the update to the stub for filter, but this should hopefully be fixed in the next release.