Bug report

Bug description:

While looking into #125618 I ran into this case.

Using the union syntax gives a ForwardRef that can't be evaluated as the class that can be evaluated gets converted into its repr via ast.Constant. Using the typing.Union class however gives a proper union object where only the undefined value is a forwardref.

• str | undefined -> ForwardRef("<class 'str'> | undefined")
• Union[str, undefined] -> str | ForwardRef("undefined")

Example:

from annotationlib import get_annotations, Format
from typing import Union

class DifferentUnions:
    attrib: str | undefined
    other_attrib: Union[str, undefined]

different_unions = get_annotations(DifferentUnions, format=Format.FORWARDREF)

print(different_unions)

Formatted Output:

{
    'attrib': ForwardRef("<class 'str'> | undefined", is_class=True, owner=<class '__main__.DifferentUnions'>), 
    'other_attrib': str | ForwardRef('undefined', is_class=True, owner=<class '__main__.DifferentUnions'>)
}

One possible solution to this is to add a create_unions attribute to the _StringifierDict. If this is True then the __or__ and __ror__ methods should create types.UnionType instances instead of calling __make_new. This is False for Format.STRING in order to keep a | b in the reproduction in that case.

I already have a branch with this approach so I can make a PR.

Doing this will break the current test_nonexistent_attribute ForwardRef test as some | obj would evaluate to ForwardRef('some') | ForwardRef('obj') instead of ForwardRef('some | obj') but I think this is probably correct and the test should be changed.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-132805 - annotationlib - Fix unions of real types and forward references. #132806
• gh-132805: annotationlib: Fix handling of non-constant values in FORWARDREF #132812