Assigning a variable to the result of an untyped method removes any existing type constraints

[rowillia]

from typing import Any, Optional

def foo() -> Optional[int]:
    return 42

raz: Any

def bar() -> int:
    x = foo()
    if isinstance(x, int):
        reveal_type(x)
        x = raz()
        reveal_type(x)
        x = x + 42
    return 0

$ python3.6 -m mypy --strict-optional test_optional_assign.py
test_optional_assign.py:11: error: Revealed type is 'builtins.int'
test_optional_assign.py:13: error: Revealed type is 'Union[builtins.int, builtins.None]'
test_optional_assign.py:14: error: Unsupported operand types for + ("Optional[int]" and "int")

I would have expected the revealed type at like 13 to be Any.

[ilevkivskyi]

I would have expected the revealed type at like 13 to be Any.

I think you mean "to be int". If yes then this is a binder problem. It may be related to #3526 and recently merged #3361. But unlike the former, this issues is probably easy to solve (I would guess just a small binder bug).

[rowillia]

@ilevkivskyi I actually did expect the type to be Any. The other case where this is biting us is below:

from typing import Any, Optional

def foo() -> Optional[int]:
    return 42

raz: Any

def bar() -> int:
    x = foo()
    if x is None:
        x = raz()
    return x

In this case raz really will set x to some integer but it's impossible to get this code to typecheck.

[ilevkivskyi]

OK, following our short discussion on gitter I think that Any is fine here. As I mentioned earlier in #3526, I am fine with the idea of rebinding to Any (especially inside unions).

[ilevkivskyi]

@msullivan I thought your PR #5629 could fix this as well, but it looks like the original example in this issue is still failing. Can we also special case this? Or this would require a more general fix?

[erictraut]

I think mypy's current behavior is correct here. The (implicitly) declared type of x is int | None. Assigning a value of type Any to x should be permitted (for reasons of gradual typing), but this assignment shouldn't change the type of x. Its type should not be narrowed to Any in this case.

[ilevkivskyi]

No mypy's behavior is not correct. Changing any precise type to Any should never create a new error. The example below works, and so the original one should too.

from typing import Any, Optional

def foo() -> Optional[int]:
    return 42

def bar() -> int:
    x = foo()
    if isinstance(x, int):
        reveal_type(x)
        x = 15
        reveal_type(x)
        x = x + 42
    return 0