Ruff rewrites Union[tuple_of_types] in potentially breaking ways

[rouge8]

After Ruff rewrites this file, Python starts throwing an error.

t.py

from typing import Union, cast


def _make_optional(cls: type) -> type:
    """Convert a type to an Optional type."""
    return cast(type, cls | None)


new_types = (int, float)

_make_optional(cast(type, Union[new_types]))

output

❯ ruff --isolated --fix --target-version py311 --select UP t.py
Found 1 error (1 fixed, 0 remaining).

rewritten

from typing import Union, cast


def _make_optional(cls: type) -> type:
    """Convert a type to an Optional type."""
    return cast(type, cls | None)


new_types = (int, float)

_make_optional(cast(type, new_types))

Unfortunately, t.py now errors out:

Traceback (most recent call last):
  File "/Users/andyfreeland/t.py", line 11, in <module>
    _make_optional(cast(type, new_types))
  File "/Users/andyfreeland/t.py", line 6, in _make_optional
    return cast(type, cls | None)
                      ~~~~^~~~~~
TypeError: unsupported operand type(s) for |: 'tuple' and 'NoneType'

version

ruff 0.0.252

[rouge8]

FWIW, pyupgrade --py311-plus t.py does not make any changes to the original t.py.

[charliermarsh]

Yeah pyupgrade never applies PEP 604 transformations to runtime types. Maybe we shouldn't either. It's come up before.

[charliermarsh]

This was another recent case: #2981.

[charliermarsh]

(So let's just change it.)