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[1.20 regression] Any is incorrectly narrowed after is comparison #21134
Description
Hi, this started happening after I upgraded to 1.20 today.
xxx: Any = object()
def func() -> None:
if 1 is not xxx:
return
reveal_type(xxx) # Revealed type is "Literal[1]"This lead to the code that utilises sentinel objects breaking:
_missing: Any = object()
def func(
alias: str | None = _missing,
id_: UUID | None = _missing,
) -> bool:
return alias is not _missing or id_ is not _missing # Non-overlapping identity check (left operand type: "UUID | None", right operand type: "str | None") [comparison-overlap]Your Environment
- Mypy version used:
mypy 1.20.0 (compiled: yes) - Mypy command-line flags: none
- Mypy configuration options from
mypy.ini(and other config files): strict - Python version used: 3.13.5
Activity
ilevkivskyi commentedon Apr 1, 2026
cc @hauntsaninja
[-]Any is incorrectly narrowed after `is` comparison[/-][+][1.20 regression] Any is incorrectly narrowed after `is` comparison[/+]ilevkivskyi commentedon Apr 1, 2026
FWIW this looks pretty bad as this is a common pattern.
Allow dangerous identity comparisons to Any typed variables
hauntsaninja commentedon Apr 1, 2026
In general, mypy will narrow away from Any, so that part is expected. That said, this is a reasonable pattern to support. Current solution I'm thinking of is #21142 , let me know if you don't like it
Allow dangerous identity comparisons to Any typed variables (#21142)
Allow dangerous identity comparisons to Any typed variables (#21142)