False-positive error when using Callable in a PEP 604 type alias in a stub file with --python-version=3.9

[AlexWaygood]

Bug Report

In a stub file, with --python-version=3.9, mypy emits false positive errors for the following type aliases:

from typing import Callable
from typing_extensions import TypeAlias

X = int | Callable[[], str | bool]
XX: TypeAlias = int | Callable[[], str | bool]
Y = int | Callable[[str | bool], str]
YY: TypeAlias = int | Callable[[str | bool], str]

For all of these, mypy's error message is:

error: Unsupported left operand type for | ("Type[str]")  [operator]

Note that this snippet works fine in a .py or .pyi file with --python-version 3.10. It's only --python-version 3.9 with a .pyi file that has the bug.

To Reproduce

1. cd into an up-to-date local clone of mypy with an editable install.
2. Copy the above snippet into a test.pyi file.
3. Run mypy test.pyi --python-version 3.9

Expected Behavior

No error should be emitted. PEP 604 syntax should be legal in stub files, even with --python-version 3.9.

Actual Behavior

False-positive errors are emitted.

Your Environment

• Mypy version used: c660354
• Mypy command-line flags: --python-version 3.9
• Python version used: 3.10

Cc. @ilevkivskyi (x-ref #12393).

[ilevkivskyi]

cc @JukkaL who recently worked on this.

[adam-grant-hendry]

Question: I have this issue in Python 3.8 in a normal .py file, but am able to get around it by 'stringifying' the type:

from __future__ import annotations

from typing import TYPE_CHECKING, Callable

from poetry.console.application import Application

if TYPE_CHECKING:
    import sys

    from poetry.poetry import Poetry

    if sys.version_info >= (3, 10):
        from typing import TypeAlias
    else:
        from typing_extensions import TypeAlias

ApplicationFactory: TypeAlias = 'Callable[[Poetry | None], Application]'
PoetryFactory: TypeAlias = 'Callable[[str | None], Poetry]'

Is this intended behavior for 3.8 with from __future__ import annotations, or should I not need to stringify the type?

[AlexWaygood]

@adam-grant-hendry that's correct behaviour from mypy. It'll fail at runtime on Python <3.10 if you don't stringify the alias, even with from __future__ import annotations. Mypy alerting you to this fact is a feature, not a bug.

from __future__ import annotations only impacts the way Python evaluates annotations at runtime; it doesn't have any impact on how Python evaluates ordinary assignments at runtime. Both ApplicationFactory and PoetryFactory are using PEP-604 syntax in assignments rather than in annotations.

[adam-grant-hendry]

that's correct behaviour from mypy

Excellent, thank you for confirming! Glad to know I did this properly 😃