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dataclasses with generated __hash__ subclassing Hashable are considered abstract #18962
Description
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 -6644214454873602895Expected 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
Activity
A5rocks commentedon Apr 24, 2025
More generally:
I suspect this is that mypy doesn't check what methods a plugin adds when looking at whether a class is abstract.
sterliakov commentedon Apr 25, 2025
No, there's no distinction between plugin-generated and regular methods for this check.
is_abstractis set after semanal, so plugin should have already been called at that point.This problem is more obvious:
mypydoes not generate__hash__on dataclasses at all. At least grepping forhashfinds no matches in dataclass plugin code.Ais accepted because we haveobject.__hash__, soAis indeed hashable from mypy perspective.Bexplicitly inherits fromHashableand thusHashable.__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.