Type narrowing fails for classes with custom `__getattr__`

[Bobholamovic]

Bug Report

When a class has a custom __getattr__ with the return type of Any, the type narrowing of mypy does not work as expected in some cases.

To Reproduce

from typing import Any, Iterator, Union
from typing_extensions import reveal_type

class A(object):
    def __getattr__(self, name: str) -> Any:
        if name == "a":
            return 1
        else:
            raise AttributeError


def func(a: Union[A, Iterator[A]]) -> None:
    if isinstance(a, A):
        reveal_type(a)
    else:
        reveal_type(a)

Expected Behavior

Revealed type is "A"
Revealed type is "typing.Iterator[A]"

Actual Behavior

Revealed type is "A"
Revealed type is "Union[A, typing.Iterator[A]]"

Your Environment

• Mypy version used: mypy 1.6.1
• Mypy command-line flags: No flags were used.
• Mypy configuration options from mypy.ini (and other config files): None.
• Python version used: Python 3.10.13

[flisboac]

I can confirm that, without __getattr__, type checking works as expected:

from typing import Any, Iterator, Union
from typing_extensions import reveal_type

class A(object):
    pass


def func(a: Union[A, Iterator[A]]) -> None:
    if isinstance(a, A):
        reveal_type(a)  # main.py:10: note: Revealed type is "__main__.A"
    else:
        reveal_type(a) # main.py:12: note: Revealed type is "typing.Iterator[__main__.A]"

I wonder why this is happening. Somehow, this reminds me of how PEP-647 (User-defined type guards) works (i.e. user-defined type narrowing is not applied to the negative case).

[Bobholamovic]

Yes, I observed that this is happening when the return type of __getattr__ is Any. In the snippet I gave, if we change -> Any to -> int, the revealed types will be correct.