Mypy incorrectly warns when passing literals as kwarg keywords

[nipunn1313]

Bug Report

from typing import Any, Dict, Literal
kw: Dict[Literal["a", "b"], Any]
def func(a, b):
    pass
func(**kw)

Leads to

main.py:8: error: Keywords must be strings

https://mypy-play.net/?mypy=latest&python=3.9&gist=582cf5e8c0a6e6a5881d391da64e1455

Expected Behavior
Expect this to be allowable - as these literals count as strings

[JelleZijlstra]

Arguably this should still be a (different) error, because the a and b args are required and a Dict[Literal["a", "b"], Any] may not contain values for a or b.

[JukkaL]

@JelleZijlstra Yes.

This should be fine, I think:

from typing import Any, Dict, Literal
kw: Dict[Literal["a", "b"], Any]
def func(a=None, b=None):
    pass
func(**kw)

[vickyhuo]

Hey @JukkaL I want to work on this. Can you please assign it to me?

[JukkaL]

@vickyhuo Done!

[Uttam-Singhh]

Can I work on it in the future?

[Soumokanti123]

@JelleZijlstra Yes.

This should be fine, I think:

from typing import Any, Dict, Literal
kw: Dict[Literal["a", "b"], Any]
def func(a=None, b=None):
    pass
func(**kw)

still it having a problem

[kamilturek]

I believe the problem is in subtype check presented below:

This case is not considered as subtype:

(Pdb) left
typing.Mapping[Union[Literal['a'], Literal['b']], Any]
(Pdb) right
typing.Mapping[builtins.str, Any]
(Pdb) is_subtype(left, right)
False

However this one is:

(Pdb) left
Union[Literal['a'], Literal['b']]
(Pdb) right
builtins.str
(Pdb) is_subtype(left, right)
True

The negative result of first presented subtype check leads to not considering Dict[Literal["a", "b"], Any] as a valid **kwarg type.

[vickyhuo]

Hi @kamilturek, I believe that is the problem as well. I'm currently still working on a fix

[hauntsaninja]

Sounds like this is just another symptom of Mapping not being covariant in its key type

[kamilturek]

@hauntsaninja What's the reason of this?

[hauntsaninja]

It's unsafe, see python/typing#445