How to represent that list is not hashable

[JukkaL]

collections.abc defines Hashable which should correspond to hashable objects. However, this is a little tricky, since hashability is not an always inherited property. Consider object vs. list:

>>> from collections.abc import *
>>> issubclass(object, Hashable)
True
>>> issubclass(list, object)
True
>>> issubclass(list, Hashable)
False

There is currently no way defined in the PEP for representing the fact that list is not hashable. We could have object be a subclass of Hashable, but without some further rules this would make list hashable as well.

Here is one potential way of specifying this in code or stubs:

class list(...)
    ...
    def __hash__(self):
        raise NotImplementedError()  # A type checker should recognize this

We could still not always check hashability reliably. For example, a variable with type object should probably be considered hashable, but it could actually be a list, which is not hashable. This is probably a very minor problem, though. Having a way of rejecting Dict[List[int], int] would be nice. We'd also need something like bounded polymorphism, though, so that we could they that a dictionary key type must a subtype of Hashtable.

Another alternative would be to not make object hashable, but since hash(object()) is okay, it doesn't sound right.

[gvanrossum]

In real code the convention (explicitly checked for by the hash() builtin) is that if you set hash to None that means the class is not hashable. Literally:

class Foo(object):
    __hash__ = None
    def __eq__(self, other): return random() < 0.5

[JukkaL]

Ah, I didn't know about that. That sounds like a good convention that should be supported by type checkers.

[vladiibine]

an abstract base class, UnHashable, which describes unhashable objects perhaps?
We could always create abc's for describing anything we wanted, and make them support the [] operator for getting an object to use in the typing system... or at least I'd hope we can extend the already suggested classes.

[gvanrossum]

I don't think we need to add this explicitly to the PEP. The __hash__ = None convention is sufficient. (It is documented here: https://docs.python.org/3/reference/datamodel.html#object.__hash__)