type not narrowed within function when it's Final

[DetachHead]

from typing import Final

foo: Final[int | None] = 1

if foo is not None:
    def bar() -> None:
        reveal_type(foo) # note: Revealed type is "Union[builtins.int, None]"

https://mypy-play.net/?mypy=master&python=3.10&flags=show-error-codes%2Callow-redefinition%2Cstrict%2Ccheck-untyped-defs%2Cdisallow-any-decorated%2Cdisallow-any-expr%2Cdisallow-any-explicit%2Cdisallow-any-generics%2Cdisallow-any-unimported%2Cdisallow-incomplete-defs%2Cdisallow-subclassing-any%2Cdisallow-untyped-calls%2Cdisallow-untyped-decorators%2Cdisallow-untyped-defs%2Cno-implicit-optional%2Cno-implicit-reexport%2Cstrict-equality%2Cwarn-incomplete-stub%2Cwarn-redundant-casts%2Cwarn-return-any%2Cwarn-unreachable%2Cwarn-unused-configs%2Cwarn-unused-ignores&gist=a8cdaa07a2217b2589777e859b6e0ff2

this is actually safe because the type is Final

[erictraut]

I think the type should be narrowed on assignment to Literal[1], so the if foo is not None conditional check shouldn't even be necessary in this example.

[DetachHead]

i know typescript behaves that way however i'm not really a fan of that behavior, i think the explicit type annotation should widen it, for the same reason that i wouldn't expect this to be narrowed just because of the return value:

def foo() -> int | None:
    return 1

[KotlinIsland]

It can be frustrating in certain circumstances:

a: object = 1
print(a + 1)  # error: Unsupported operand types for + ("object" and "int")

a = "amoogus"
print(a + "SUS")

And forces you to write code that you normally wouldn't:

a: object 
a = 1
print(a + 1)  # no error: mypy happy now 😀

a = "amoogus"
print(a + "SUS")