8000 Incorrect inference for types with `__eq__` and no `__hash__` · Issue #18622 · python/mypy · GitHub
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Incorrect inference for types with __eq__ and no __hash__ #18622

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jorenham opened this issue Feb 6, 2025 · 0 comments
Open

Incorrect inference for types with __eq__ and no __hash__ #18622

jorenham opened this issue Feb 6, 2025 · 0 comments
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bug mypy got something wrong topic-runtime-semantics mypy doesn't model runtime semantics correctly

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@jorenham
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jorenham commented Feb 6, 2025

problem

class A:
    def __eq__(self, o: object, /) -> bool:
        return self is o

reveal_type(A().__hash__)  # def () -> builtins.int

https://mypy-play.net/?mypy=latest&python=3.13&flags=strict%2Clocal-partial-types&gist=2b23624b1df772175b70c08e36a6e912

yet

>>> class A:
...     def __eq__(self, o: object, /) -> bool:
...         return self is o
...         
>>> type(A.__hash__)
<class 'NoneType'>

solution

iff. class A, and only class A (i.e. excluding any base classes)

  • implements __eq__, and
  • does not implement __hash__

infer A.__hash__: ClassVar[None]
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