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unreachable false positive when using isinstance on class with __call__ when function parameter uses ParamSpec #16024

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DetachHead opened this issue Sep 3, 2023 · 2 comments
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unreachable false positive when using isinstance on class with __call__ when function parameter uses ParamSpec #16024

DetachHead opened this issue Sep 3, 2023 · 2 comments
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bug mypy got something wrong topic-protocols topic-reachability Detecting unreachable code

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@DetachHead
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from typing import Callable, ParamSpec

P = ParamSpec("P")


class Foo:
    def __call__(self) -> None:
        ...


def foo(fn: Callable[P, None]) -> None:
    if isinstance(fn, Foo):
        print("reached") # error: Statement is unreachable  [unreachable]


foo(Foo())

playground

in this example, that line is always reached at runtime
@DetachHead DetachHead added the bug mypy got something wrong label Sep 3, 2023
@AlexWaygood AlexWaygood added topic-protocols topic-reachability Detecting unreachable code labels Sep 3, 2023
@erictraut erictraut mentioned this issue Sep 3, 2023
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@DetachHead
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minified further:

from typing import Callable

def foo(fn: Callable[[], object]) -> None:
    if isinstance(fn, type):
        print(fn)

@sterliakov
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Works since v1.11.0, resolved in #17389.

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@hauntsaninja @sterliakov @DetachHead @AlexWaygood