Incorrect error for unions syntax in comments and strings (Python 3.9)

[relikd]

Bug Report

Minimal example python code is attached below. mypy output is:

main.py:4: error: X | Y syntax for unions requires Python 3.10  [syntax]
main.py:12: error: X | Y syntax for unions requires Python 3.10  [syntax]
Found 2 errors in 1 file (checked 1 source file)

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.9&gist=03da1f2a4b60eb95088e81958066deb5

just in case, here a copy of the gist

#!/usr/bin/env python3
class C3:
    futureDef: 'C2'
    two: 'str|None'


class C2:
    pass


class C1:
    ONE = None  # type: str|None

    def begin(self) -> None:
        C1.ONE = 'a'

Expected Behavior

This file should produce no errors

Actual Behavior

Both errors are incorrect, as both types are provided in a 3.8 safe way (encased in a string or as a comment)

Your Environment

• Mypy version used: 1.17.1 (latest)
• Python version used: 3.9

[JawadAliAI]

Mypy currently raises X | Y syntax for unions requires Python 3.10 when string-quoted type hints include the | operator under Python < 3.10 (e.g., "str | None"). Since these annotations are provided as strings (permitted forward references under PEP 563), they should not be subject to version-specific syntax restrictions. For compatibility with Python 3.9, mypy should treat such cases as plain forward references rather than parsing them with Python 3.10+ rules.

As a workaround, Optional[str] or Union[str, None] can be used, but the expected behavior is that quoted annotations using | should be accepted without error.

I would be happy to work on this issue if it is considered valid. Please let me know if I can proceed with a pull request.

[relikd]

Any progress on the issue? I feel sad that people are willing to work on this but no one is giving the go signal.

[JelleZijlstra]

The best way to move something like this forward is to submit a PR. Unfortunately maintainer time on mypy is limited, so it may take a while before a maintainer is able to review and merge a PR.

I would consider this issue valid, but given that Python 3.9 is about to go out of support in about two days, realistically the outcome may well be that mypy drops support for 3.9 before this gets fixed.

[relikd]

Thank you for your insight. I'd understand if that were to happen though it would still be a shame :( macOS (15) is still shipping with 3.9 as default. I know Apple is here to blame, but still it would be nice to use software out of the box.

[Avasam]

Looks like mypy 2.0 has dropped support for even parsing Python 3.9:

• Drop support for --python-version 3.9 #21243
• https://mypy-lang.blogspot.com/2026/05/mypy-20-relased.html#:~:text=Drop%20Support%20for%20Targeting%20Python%203.9