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[3.14] annotationlib - Union '|' syntax and typing.Union[...] generate different forward references. #132805
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Good catch. I think this is a more general problem though, for example:
>>> class X: ... y: foo[str, undefined] ... >>> get_annotations(X, format=Format.FORWARDREF) {'y': ForwardRef("foo[<class 'str'>, undefined]", is_class=True, owner=<class '__main__.X'>)}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.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.- added a commit that references this issue
on May 4, 2025 - added a commit that references this issue
on May 4, 2025
Bug report
Bug description:
While looking into #125618 I ran into this case.
Using the union syntax gives a
ForwardRefthat can't be evaluated as the class that can be evaluated gets converted into itsreprviaast.Constant. Using thetyping.Unionclass 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:
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_unionsattribute to the_StringifierDict. If this isTruethen the__or__and__ror__methods should createtypes.UnionTypeinstances instead of calling__make_new. This isFalseforFormat.STRINGin order to keepa | bin 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_attributeForwardRef test assome | objwould evaluate toForwardRef('some') | ForwardRef('obj')instead ofForwardRef('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
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