Type of global variable is not narrowed in function scopes
#311
Comments
global variable is not narrowed in function scopes
|
Thank you for reporting this. The problem here is that narrowing on x: int | None = 1
def reset_x() -> None:
global x
x = None
def f() -> None:
global x
if x is not None:
reset_x()
reveal_type(x) # ?Other type checkers such as mypy and pyright report |
This is true, and it's also possible that code from another thread could mutate the global in a concurrent program. The unsafety here also applies to narrowing of attributes and subscript expressions, however. You already linked to #164, but just to spell it out: class Foo:
x: int | None = 1
def reset_x(t: Foo) -> None:
t.x = None
def f(t: Foo) -> None:
if t.x is not None:
reset_x(t)
reveal_type(x) # ?But we intend to support narrowing for attributes and subscripts nonetheless, since a lot of real-world code relies on it, users strongly expect it to work, and the practical experience of existing type checkers has been that the theoretical unsoundness here rarely causes practical issues in Python code. I think narrowing for |
Summary
https://play.ty.dev/f0b802a2-9e21-438d-8946-6386a45b45b5
We already checked that if it's None we assign a value to it, so the return value can't be possibly None anymore
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