Hashable doesn't work

[miedzinski]

Typechecking Hashable seems broken.

from typing import Hashable


def f(x: Hashable):
    pass


f(0)
f('x')

$ mypy mypy.py
mypy.py:8: error: Argument 1 to "f" has incompatible type "int"; expected "Hashable"
mypy.py:9: error: Argument 1 to "f" has incompatible type "str"; expected "Hashable"
$ mypy -V
mypy 0.4.3-dev

[gvanrossum]

There are two solutions here; mypy could learn that Hashable is related to
hash, or we could inherit int, str etc. from it explicitly in typeshed.

--Guido (mobile)

[miedzinski]

Second option doesn't fix an issue with classes that have __hash__ defined.

Is there any chance this will be put somewhere in roadmap?

[gvanrossum]

I agree (1) isn't very attractive. In terms of roadmap it would be part of "structural type checking". python/typing#11

Maybe we should just close it here? There's already lots of discussion at that issue.

[JukkaL]

object is hashable, so just inheriting Hashable or is not a good approach, as then every type would be hashable. I think that mypy would have to know about the __hash__ = None idiom for marking a type as not hashable. This clearly breaks Liskov substitutability but mypy could still catch some clear violations such as using a list as a dictionary key even if it wouldn't be sound.

[pkch]

In addition, currently deriving from Hashable breaks the normal hashability because typing.Hashable is a non-virtual subclass of collections.abc.Hashable, and the latter disables hashability by overriding object.__hash__ with a virtual method __hash__:

from typing import Hashable
class X(Hashable): ...
x = X() # Can't instantiate abstract class X with abstract methods

This is something that maybe can be documented or fixed in the process of future upgrades to Hashable.

[ilevkivskyi]

Superseded by python/typeshed#2148