Type Narrowing: if val == Literal should always narrow to Literal

[CarliJoy]

Feature

Currently mypy already type narrows literals in if

def works(abc: Literal["a", "b", "c"]):
    a: Literal["a"] | None = None
    if abc == "a":
        a = abc

def works_not(anystring: str):
    a: Literal["a"] | None = None
    if anystring == "a":
        a = anystring  # expression has type str

More Complete MyPy Play

Pitch

See #16813 for an example why this is useful.
In general working with TypedDicts in for loop this helps a lot reducing duplicate code / keys.

[JelleZijlstra]

Agree. There's some risk this could lead to new false positives in cases involving invariance, but on balance this is almost certainly a positive.

[erictraut]

FWIW, pyright applies narrowing in this case. Pyright playground. I haven't observed false positives in cases involving invariance, but I agree that could be a small concern if code bases are relying on mypy's current behavior.

[CarliJoy]

Actually the full example works already in pyright as expected