Description

DavidCEllis

opened on Apr 22, 2025

Issue body actions

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

Activity

1. DavidCEllis

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Apr 22, 2025
2. sobolevn

   added

   topic-typing

   on Apr 22, 2025
3. JelleZijlstra commented on Apr 22, 2025

   

   JelleZijlstra

   on Apr 22, 2025

   Member

   More actions

   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_ast where we currently return ast.Constant(). Instead, we could pick a unique name, emit an ast.Name referring 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 like ForwardRef("foo[$name1, undefined]", known_values={"$name1": str}). I can look into implementing that.
4. DavidCEllis commented on Apr 22, 2025

   

   DavidCEllis

   on Apr 22, 2025

   ContributorAuthor

   More actions

   Ah, yes you're right. Sounds similar to what dataclasses needs to do to handle the values it needs for default values/types in exec. I'd only looked at exists[str, undefined] and not the case where the base object was undefined too.
5. picnixz

   added

   extension-modulesC modules in the Modules dirC modules in the Modules dir

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   3.14bugs and security fixesbugs and security fixes

   and removed

   extension-modulesC modules in the Modules dirC modules in the Modules dir

   on Apr 24, 2025
6. JelleZijlstra

   added a commit that references this issue on May 4, 2025

   gh-132805: annotationlib: Fix handling of non-constant values in FORW…

   c8f233c
7. JelleZijlstra

   closed this as completedin #132812on May 4, 2025
8. diegorusso

   added a commit that references this issue on May 4, 2025

   Merge remote-tracking branch 'origin/main' into call-len

   c03babc
9. Pranjal095

   added a commit that references this issue on Jul 12, 2025

   pythongh-132805: annotationlib: Fix handling of non-constant values i…

   73a12cb

Issue actions