Repository navigation
Type-narrowing Union with Literal using comparison to Literal variable #16465
Description
Activity
I would say that this is indeed a bug. It won't be a bug in the positive case (as a class that redefines
__eq__MAY be equal to a string), but it should definitely narrow in the negative case. Here you can see that the problem manifest itself depending on if you redefined__eq__or not:from datetime import datetime from typing import Final, Literal Unspecified = Literal['unspecified'] UNSPECIFIED: Final[Unspecified] = 'unspecified' class KK(): def kk(self): pass class KK2(): def kk(self): pass def __eq__(self, other): return False def do_thing(x: datetime | Unspecified) -> None: if x != UNSPECIFIED: print(x.utcnow()) # Unexpected error! def do_thing_2(x: int | Unspecified) -> None: if x != UNSPECIFIED: print(x.real) # No prob def do_thing_3(x: KK | Unspecified) -> None: if x != UNSPECIFIED: print(x.kk) # No prob def do_thing_4(x: KK2 | Unspecified) -> None: if x != UNSPECIFIED: print(x.kk) # Unexpected error!
Reacted by Garrett ReynoldsCopying over my smaller reproducer from #18569:
from typing import Literal class A: def __eq__(self, other: object) -> bool: # necessary return isinstance(other, A) def f(v: A | Literal["text"]) -> A | None: if v == "text": reveal_type(v) # N: Revealed type is "Union[__main__.A, Literal['text']]" return None else: reveal_type(v) # N: Revealed type is "Union[__main__.A, Literal['text']]" return v
Reacted by Garrett Reynolds- added a commit that references this issue
on Jan 27, 2026
Bug Report
Strangely, type-narrowing for unions including a Literal works for
intbut notdatetimeTo Reproduce
Mypy Playground. Copied here for convenience:
Expected Behavior
No errors.
Actual Behavior
Has errors:
Your Environment
mypy.ini(and other config files): <none>