Non-hashable types should declare __hash__: None in typeshed

[ilevkivskyi]

Currently this passes mypy:

from typing import Hashable

def f(x: Hashable):
    pass

f([])

A possible solution may be to add __hash__: None to non-hashable types like list.

[JelleZijlstra]

I tried doing this in #2221, but I get errors like:

stdlib/3/builtins.pyi:515: error: Incompatible types in assignment (expression has type "None", base class "object" defined the type as "Callable[[object], int]")

We probably need some more special casing in mypy first.

[ilevkivskyi]

We probably need some more special casing in mypy first.

Yes. This particular one can be fixed by changing definition on object to Optional[...], but there are other errors, for example one can define a hashable subclass of dict. Simplest solution is to just silence the error if is because of a dunder with None type in superclass.

[gvanrossum]

We probably need some more special casing in mypy first.

Can you file a separate mypy issue for that? Having that fixes is a prerequisite for fixing this (and hence for #2221).

[srittau]

mypy issue is now python/mypy#4266.

[ilevkivskyi]

Note that this can also cause false positives, see python/mypy#7328.

Honestly, I don't think having few # type: ignores is a big problem, maybe we should revive #2221 to mitigate this until we implement some special-casing in mypy?

[srittau]

So from what I understand we would add __hash__ = None # type: ignore to unhashable classes? I'm fine with that. (Although not needing # type: ignore would be better, of course.)

[ilevkivskyi]

So from what I understand we would add __hash__ = None # type: ignore to unhashable classes?

Yes, or maybe better __hash__: None # type: ignore (to have a consistent style, plus IIRC pytype likes the latter more).