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[3.14] annotationlib - Union '|' syntax and typing.Union[...] generate different forward references. #132805
Description
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
Activity
JelleZijlstra commentedon Apr 22, 2025
Good catch. I think this is a more general problem though, for example:
A more general approach to fix this would be to change the case in
__convert_to_astwhere we currently returnast.Constant(). Instead, we could pick a unique name, emit anast.Namereferring to that name, and make the ForwardRef remember a dictionary mapping such names to the original value. So in this case, we'd get something likeForwardRef("foo[$name1, undefined]", known_values={"$name1": str}). I can look into implementing that.DavidCEllis commentedon Apr 22, 2025
Ah, yes you're right. Sounds similar to what
dataclassesneeds to do to handle the values it needs for default values/types in exec. I'd only looked atexists[str, undefined]and not the case where the base object was undefined too.gh-132805: annotationlib: Fix handling of non-constant values in FORW…
Merge remote-tracking branch 'origin/main' into call-len
pythongh-132805: annotationlib: Fix handling of non-constant values i…