typing.Annotated fails for functions returning UUIDs

[dave-shawley]

Bug report

Bug description:

typing.Annotated tries to set an attribute on the return value of calling an annotated callable. This fails when the returned object is immutable. I believe that adding TypeError to the except clause in typing. _BaseGenericAlias.__call__ will fix the problem. I'm not sure if there are more cases of the same pattern or not.

cpython/Lib/typing.py

Lines 1128 to 1131 in 17689e3

	try:
	result.__orig_class__ = self
	except AttributeError:
	pass

Example

import typing
import uuid

class MyAnnotation:
    def __init__(self, **properties) -> None:
        self.properties = properties

def uuid_from_str(s: str) -> uuid.UUID:
    return uuid.UUID(f'urn:uuid:{s}')

coercion = typing.Annotated[uuid_from_str, MyAnnotation(type='str', format='uuid')]
coercion('00000000-0000-0000-0000-000000000000')

Result

Traceback (most recent call last):
  File "/Users/.../foo.py", line 12, in <module>
    coercion('00000000-0000-0000-0000-000000000000')
  File "/Users/.../lib/python3.12/typing.py", line 1142, in __call__
    result.__orig_class__ = self
    ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/.../lib/python3.12/uuid.py", line 278, in __setattr__
    raise TypeError('UUID objects are immutable')
TypeError: UUID objects are immutable

CPython versions tested on:

3.9, 3.10, 3.11, 3.12

Operating systems tested on:

macOS

Linked PRs

• gh-115165: Fix Annotated for immutable types #115213
• [3.12] gh-115165: Fix typing.Annotated for immutable types (GH-115213) #115227
• [3.11] gh-115165: Fix typing.Annotated for immutable types (GH-115213) #115228

[AlexWaygood]

Thanks for the report!

Arguably there are two bugs here:

1. Why is the uuid module raising TypeError from a __setattr__ method? That seems like an antipattern; it should probably be raising AttributeError. We probably can't change that now, though, for backwards-compatibility reasons
2. Yes, the typing module should indeed be more careful here. Feel free to send a PR — I think except Exception might actually be better here than just adding TypeError to the except clause. Otherwise we'll just get this bug reported again when somebody tries to use Annotated with a class that raises e.g. RuntimeError (or whatever) from its __setattr__ method

[AlexWaygood]

Thanks for the report, and the fix!