Nested functions don't take type restrictions into account

[gvanrossum]

Example:

from typing import *

def f(a: Union[int, str]) -> int:
    if isinstance(a, str):
        return -1
    def inner() -> int:
        return a+1  # E: Unsupported operand types for + ("Union[int, str]" and "int"
    return inner()

Note that when inner() is defined, and also when it's called, a can only be an int. But we still get an error.

I ran into this when checking some real-world code against strict optional -- there the union was Optional[int] and the type restriction was if a is not None, but it's the same problem.

[ddfisher]

I think this is quite difficult to do correctly. This is correct because f exits before inner is defined, but wouldn't be correct for other local information about types. For example:

from typing import *

def f(a: Union[int, str]) -> int:
    if isinstance(a, str):
        return -1
    else:
        def inner() -> int:
            return a+1  # E: Unsupported operand types for + ("Union[int, str]" and "int"
    a = "foo"
    return inner()

The binder doesn't keep track of this information at the moment -- I'm not sure how hard it would be to add, but probably not super easy.

[gvanrossum]

I see. I'm fine with closing this then.