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ralismark opened this issue Apr 24, 2025 · 2 comments · May be fixed by #19043
Open

dataclasses with generated __hash__ subclassing Hashable are considered abstract #18962

ralismark opened this issue Apr 24, 2025 · 2 comments · May be fixed by #19043
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bug mypy got something wrong topic-dataclasses

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@ralismark
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Bug Report

Dataclasses that are hashable (e.g. frozen=True) that extend any abc/protocol requiring __hash__ (e.g. collections.abc.Hashable) is considered abstract and gives an error when trying to be instantiated.

In addition to frozen=True/Hashable, you also get the same error with unsafe_hash=True, as well as using your own protocol or abc that requires __hash__.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.12&gist=390504f01055e489b12b388eec1de256

from dataclasses import dataclass
from collections.abc import Hashable


@dataclass(frozen=True)
class A:
    a: int


@dataclass(frozen=True)
class B(Hashable):
    a: int


a: Hashable = A(1)  # ok
b = B(1)  # error: Cannot instantiate abstract class "A" with abstract attribute "__hash__"  [abstract]

print(hash(b))  # prints -6644214454873602895

Expected Behavior

This works at runtime -- running the script prints the hash, demonstrating that it has __hash__ defined.

Actual Behavior

main.py:16: error: Cannot instantiate abstract class "B" with abstract attribute "__hash__"  [abstract]

Your Environment

  • Mypy version used: 1.15.0
  • Mypy command-line flags: N/A
  • Mypy configuration options from mypy.ini (and other config files): N/A
  • Python version used: 3.12
@ralismark ralismark added the bug mypy got something wrong label Apr 24, 2025
@A5rocks
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A5rocks commented Apr 24, 2025

More generally:

from abc import ABC, abstractmethod
import dataclasses

class Hashable(ABC):
    @abstractmethod
    def __hash__(self) -> int:
        ...

@dataclasses.dataclass(frozen=True)
class X(Hashable):
    a: int

X(4)

I suspect this is that mypy doesn't check what methods a plugin adds when looking at whether a class is abstract.

@sterliakov
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No, there's no distinction between plugin-generated and regular methods for this check. is_abstract is set after semanal, so plugin should have already been called at that point.

This problem is more obvious: mypy does not generate __hash__ on dataclasses at all. At least grepping for hash finds no matches in dataclass plugin code. A is accepted because we have object.__hash__, so A is indeed hashable from mypy perspective. B explicitly inherits from Hashable and thus Hashable.__hash__ is the highest MRO entry, and it's abstract.

Probably we should just generate __hash__ on dataclasses. This should be easy to do, dunder-hash is simple and non-generic.

@sterliakov sterliakov linked a pull request May 6, 2025 that will close this issue
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Generate dataclasses __hash__ according to runtime semantics sterliakov/mypy
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@ralismark @A5rocks @sterliakov