str not detected as Hashable

[corynezin]

This is probably a duplicate of #1746 - but I don't see a resolution

• Are you reporting a bug, or opening a feature request?

• Not sure if it's a bug or intended

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

from typing import Hashable, Dict

def foo(d: Dict[Hashable, int]):
  ...

d: Dict[str, int] = {'a': 1}
foo(d)

• What is the actual behavior/output?
  test.py:7: error: Argument 1 to "foo" has incompatible type "Dict[str, int]"; expected "Dict[Hashable, int]"
• What is the behavior/output you expect?
  Success: no issues found in 1 source file
• What are the versions of mypy and Python you are using?
  Do you see the same issue after installing mypy from Git master?
  mypy 0.761
• What are the mypy flags you are using? (For example --strict-optional)
  None
• If mypy crashed with a traceback, please paste
  the full traceback below.

(You can freely edit this text, please remove all the lines
you believe are unnecessary.)

[JelleZijlstra]

This due to invariance; see https://mypy.readthedocs.io/en/latest/common_issues.html#invariance-vs-covariance. (Although Hashable is generally not very well-supported anyway.)

[jamesmcm]

Is there a solution to this aside from just ignoring the type check though?

Every str should be Hashable, so why doesn't this work?

[Michael0x2a]

@jamesmcm -- mypy is reporting a legitimate issue with the above code. Basically, it would be legal for foo to do d[1] = 2 since ints are hashable, but that would end up breaking the caller since the keys are now no longer strings.

The docs linked above discuss this problem and some workarounds. The quickest one for the above example would be to switch to having foo accept a Mapping[Hashable, int], which is a read-only version of a dict, which in turn means the key type is allowed to be covariant instead of invariant.

Or to rephrase, having the data structure be read-only/disallowing foo from mutating the input means we can be more permissive about what we can pass in.

[JelleZijlstra]

@Michael0x2a Mapping is actually also invariant in the key type, see https://github.com/python/typeshed/blob/master/stdlib/3/typing.pyi#L400.