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graingert opened this issue Mar 10, 2022 · 3 comments
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warn redundant casts doesn't work with Protocol.__call__ #12328

graingert opened this issue Mar 10, 2022 · 3 comments
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bug mypy got something wrong topic-casts topic-protocols

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@graingert
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graingert commented Mar 10, 2022 •
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To Reproduce

https://mypy-play.net/?mypy=latest&python=3.10&flags=strict%2Cwarn-redundant-casts&gist=bc9999020b0fb32610c0c3ec99a8568c

from typing import Protocol, cast

class FooType(Protocol):
    def __call__(self, ham: str) -> str:
        pass

def foo(ham: str) -> str:
    pass


v = cast(FooType, foo)
u: FooType = foo

Expected Behavior
I was expecting v = cast(FooType, foo) to work about the same as v: FooType = foo # type: ignore[assignment] with warn redundant ignores

Your Environment

See mypy-play
@graingert graingert added the bug mypy got something wrong label Mar 10, 2022
@graingert
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graingert commented Mar 10, 2022 •
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It seems even very simple redundant casts are not detected:

from typing import cast

class Foo:
    pass

class Bar(Foo):
    pass

v = cast(Foo, Bar())

https://mypy-play.net/?mypy=latest&python=3.10&flags=strict%2Cwarn-redundant-casts&gist=c166276e9623ced551d45e7a74d31510

@jhance
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jhance commented Mar 11, 2022

Is the behavior with respect to subtyping documented?

@graingert
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The docs refer to casts that can be safely removed:

https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-warn-redundant-casts

This flag will make mypy report an error whenever your code uses an unnecessary cast that can safely be removed.

@sterliakov sterliakov mentioned this issue Apr 20, 2025
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4 participants
@jhance @graingert @sterliakov @AlexWaygood