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Type Narrowing: if val == Literal should always narrow to Literal #16819
Description
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 strPitch
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.
Activity
JelleZijlstra commentedon Jan 25, 2024
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 commentedon Jan 25, 2024
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 commentedon Jan 26, 2024
Actually the full example works already in pyright as expected