Mypy doesn't catch unhashable literals in hashdependant literals.

[mental32]

context on the python-dev mailing list

As per Guido and Nicks suggestions I am opening this issue here :D

"""foo bar."""

from typing import Set, Dict

SOME = {{0}}
print(SOME)


def green_eggs() -> Set[Set[int]]:
    """foo."""
    return {{1}}


def and_ham() -> Dict[Set[int], int]:
    """bar."""
    return {{1}: 0}

Mypy reports this as fine with no errors.
It should detect and throw when encountering {{1}: 0} or Dict[Set[int], int] or {{1}} or {[1]} since these are literals whose types are not hashable therefore they may not be used in types that require members be hashable i.e. Dict or Set.

I'm using mypy 0.750 and Python 3.8.0 (default, Oct 23 2019, 18:51:26) [GCC 9.2.0] on linux

If needed I'm happy to attempt to PR this myself :)

[JelleZijlstra]

This is basically a duplicate of #2455, although the example in this issue is a bit more foundational.

The reason why this has been open for so long is that the hashability protocol in Python behaves a bit weirdly: all objects are hashable by default, except that certain objects explicitly mark themselves as not hashable. Catching this sort of thing in mypy probably requires some special casing.

[ilevkivskyi]

Yeah, I think this is a dupe.