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mypy doesn't narrow Final objects in nested contexts #19080
Description
Bug Report
I have a global constant I've marked with Final. In a class definition, I use this constant to swap out implementations of a method. While inside a type guarding block, but outside the method definition, the type of the Final object is narrowed, but inside of the method it is not narrowed.
To Reproduce
https://mypy-play.net/?mypy=latest&python=3.12&gist=2d4c97cffea1dd6710bf5cc7283f7290
from typing import *
def _init() -> None | str:
return None
RESOLVE_X: Final = _init()
class Example:
if RESOLVE_X is not None:
reveal_type(RESOLVE_X) # note: Revealed type is "builtins.str"
def __str__(self) -> str:
return RESOLVE_X # error: Incompatible return value type (got "str | None", expected "str") [return-value]
else:
def __str__(self) -> str:
return "example"Expected Behavior
Inside the __str__ definition in the if RESOLVE_X is not None: block, RESOLVE_X should stay narrowed as it cannot be reassigned and have its type re-widened in the future.
Actual Behavior
main.py:12: note: Revealed type is "builtins.str"
main.py:15: error: Incompatible return value type (got "str | None", expected "str") [return-value]
Found 1 error in 1 file (checked 1 source file)
Your Environment
Mypy version 1.15.0.
Python 3.12.
Activity
Admit that Final variables are never redefined (#19083)
ilevkivskyi commentedon Apr 15, 2026
Support for this is being removed in #21241. Re-opening in case we will decide to support this again later.
ilevkivskyi commentedon Apr 15, 2026
A recommended workaround is to use
assert RESOLVE_X is not Nonein the function body.ilevkivskyi commentedon Apr 16, 2026
Another possible (and in some cases better) workaround proposed in #21241 is