`PLW1641`: false positive

[chirizxc]

The current eq-without-hash (PLW1641) rule warns whenever a class defines __eq__ without defining __hash__. However, this creates false positives for legitimate use cases.

Not all classes with __eq__ are intended to be used as dictionary keys or set members. For mutable objects, having __hash__ = None is actually the correct behavior according to Python's data model:

Consider the inverse check instead: warn when a class defines __hash__ (and it's not None) but doesn't define __eq__. This would catch cases where the hash/equality contract is violated: objects that are equal must have the same hash value.

Playground*

[ntBre]

I think this might be a duplicate of, or at least closely related to, #18821. As noted there, I don't think there's a good way for us to detect a mutable object or otherwise suppress the error. Explicitly setting __hash__ = None seems like the easiest way to indicate your intention to Ruff. This comment mentions that this could lead to other linting errors, but I'm not seeing any on the playground, at least with my config there.