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glyph opened this issue May 5, 2023 · 0 comments · Fixed by #15913
Closed

callable protocols cannot infer the type of their own self #15188

glyph opened this issue May 5, 2023 · 0 comments · Fixed by #15913
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bug mypy got something wrong

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@glyph
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glyph commented May 5, 2023

Bug Report

Methods on protocols with a __call__ method cannot infer the type of self
as callable in other methods.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.11&flags=strict&gist=7234281168a0762dd9a81b1f10d8a96a

from typing import Callable, ParamSpec, Protocol, TypeVar

Params = ParamSpec("Params")
Result = TypeVar("Result", covariant=True)

class FancyMethod(Protocol):

    def __call__(self, arg1: int, arg2: str) -> list[float]:
        """
        Calling me results in a very specific signature.
        """

    def returnMe(self: Callable[Params, Result]) -> Callable[Params, Result]:
        ...

def externalGet(self: Callable[Params, Result]) -> Callable[Params, Result]:
    return self

Expected Behavior

I would expect that this would be inferred in returnMe the same way that it would be in externalGet.

Actual Behavior

main.py:13: error: The erased type of self "def (*Params.args, **Params.kwargs) -> Result`-2" is not a supertype of its class "__main__.FancyMethod"  [misc]
@glyph glyph added the bug mypy got something wrong label May 5, 2023
@glyph glyph mentioned this issue May 5, 2023
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@ilevkivskyi ilevkivskyi mentioned this issue Aug 26, 2023
Merged
hauntsaninja pushed a commit that referenced this issue Sep 14, 2023
@ilevkivskyi
Lenient handling of trivial Callable suffixes (#15913)
f41e24c 
Fixes #15734
Fixes #15188
Fixes #14321
Fixes #13107 (plain Callable was
already working, this fixes the protocol example)
Fixes #16058

It looks like treating trivial suffixes (especially for erased
callables) as "whatever works" is a right thing, because it reflects the
whole idea of why we normally check subtyping with respect to an e.g.
erased type. As you can see this fixes a bunch of issues. Note it was
necessary to make couple more tweaks to make everything work smoothly:
* Adjust self-type erasure level in `checker.py` to match other places.
* Explicitly allow `Callable` as a `self`/`cls` annotation (actually I
am not sure we need to keep this check at all, since we now have good
inference for self-types, and we check they are safe either at
definition site or at call site).
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Lenient handling of trivial Callable suffixes ilevkivskyi/mypy
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@glyph