stubgen: use X | Y / X | None instead of Union[X, Y] / Union[X, None]

[eggplants]

Feature

I would like to let stubgen use X | Y / X | None instead of Union[X, Y] / Union[X, None].

Pitch

Now:

# Python 3.10.4, mypy 0.960
$ cat test.py
import random


def a() -> int | str:
    if random.random() > 0.5:
        return 1
    else:
        return "hey!"

def b() -> str | None:
    if random.random() > 0.5:
        return "hey!"

$ stubgen test.py
Processed 1 modules
Generated out/test.pyi

$ cat out/test.pyi
def a() -> Union[int, str]: ...
def b() -> Union[str, None]: ...

Expect:

def a() -> int | str: ...
def b() -> str | None: ...

[eggplants]

Anyway, it is weird that from typing import Union is not inserted by stubgen...

[JelleZijlstra]

@eggplants if stubgen doesn't import Union when it should, please open a separate issue with a reproducer. We should definitely fix that, and I suspect it'll be easy.

I'm a little more uncertain about this issue, since mypy still has a few bugs where it doesn't accept |, and it would be bad if stubgen generated stubs that mypy itself rejects.

[eggplants]

@JelleZijlstra Thank you. Created: #12929

[AlexWaygood]

I'm a little more uncertain about this issue, since mypy still has a few bugs where it doesn't accept |, and it would be bad if stubgen generated stubs that mypy itself rejects.

We could safely teach stubgen to use PEP 604 in non-runtime contexts. But that might be more complicated than it's worth; it might be better to wait until mypy has full PEP-604 support.