Type inference does not work for inherited Literal properties

[antonagestam]

I expected mypy to be able to infer that the literal in the subtype is a subtype of the property in its ancestor. Should this be allowed? I didn't find anything in the PEP defining the behavior here, but it seems like a really useful thing to be able to do.

from typing_extensions import Literal
from typing import Union


HTTPMethodName = Union[
    Literal['GET'],
    Literal['POST'],
]


class A:
    method: HTTPMethodName


class B(A):
    # works if the subclass also annotates the property
    method: HTTPMethodName = 'POST' 


class C(A):
    # error: Incompatible types in assignment (expression has type "str", base class "A" defined the type as "Union[Literal['GET'], Literal['POST']]")
    method = 'POST'

setup.cfg:

[mypy]
ignore_missing_imports = True
strict_optional = True
no_implicit_optional = True
check_untyped_defs = True
disallow_incomplete_defs = True

$ mypy --version
mypy 0.710+dev.b53dae73eebf72e3fe1f868dfa9ad008204c5701

[Michael0x2a]

Yeah, this does seem like a bug -- I suspect we're not taking the parent type/existing type into account when determining if we're in a literal context while performing type inference.

I'll dig a little deeper into this later today.

[ilevkivskyi]

@Michael0x2a I first wanted to close this as a duplicate of #4547, but it is up to you whether to track the effect on literal types separately.

@antonagestam You can use Literal['GET', 'POST'] instead of the union.

[hiaselhans]

mypy also fails to infer the type as literal for this simple example:

from typing import Literal


def test(a: Literal["a", "b"]) -> None:
    return a + "-test"


def test2() -> None:
    x="a"
    test(x) # error: Argument 1 to "test" has incompatible type "str"; expected "Literal['a', 'b']"

[antonagestam]

@hiaselhans That's a quite different issue, and I think it's to be expected. FWIW, it works if you make x final.

[erictraut]

This bug appears to have been fixed. The error is no longer emitted by the latest version of mypy.