Presence of future annotation import causes improper application of PEP 604 for Python 3.7

Given these file contents:

# foo.py

from __future__ import annotations

from typing import Optional


def some_function(some_arg: Optional[str] = None):
    ...

Running the following command:

$ pyupgrade --py37-plus

Rewrites the file contents to:

# foo.py

from __future__ import annotations

from typing import Optional


def some_function(some_arg: str | None = None):
    ...

Which to my understanding is not valid before Python 3.10. Note that if the from __future__ import annotations line is not present initially, the file remains unchanged.

This is the case on at least versions 2.29.1 and 2.31.0.

[MarcoGorelli]

It isn’t valid in Python 3.7 regardless

it is valid if you have from __future__ import annotations:

(venv37) marcogorelli@OVMG025 tmp % cat t.py 
# from __future__ import annotations
a: str | None
(venv37) marcogorelli@OVMG025 tmp % python3.7 t.py 
Traceback (most recent call last):
  File "t.py", line 2, in <module>
    a: str | None
TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'
(venv37) marcogorelli@OVMG025 tmp % vim t.py 
(venv37) marcogorelli@OVMG025 tmp % cat t.py 
from __future__ import annotations
a: str | None
(venv37) marcogorelli@OVMG025 tmp % python3.7 t.py

I took these listed conditions to be ANDed together, but seems instead to behave as an OR currently.

That’s right, it’s OR. I think this was brought up before but the maintainer thought it was clear enough as it is

[asottile]

you don’t respect it though, you’re here making noise and telling people how to use the tool