Enum misuse causes unhelpful error

The traceback names cattrs/converters.py and _unstructure_enum, with the generated unstructure_Site path showing where the failure surfaces. First reproduce the reduced attrs example and inspect how enum unstructuring is dispatched. The issue does not define the desired replacement behavior, so confirm the expected developer-facing outcome before deciding what test or changed failure would count as done.

• cattrs version: 24.1.2
• Python version: 3.11
• Operating System: Linux

Description

Use an enum's value as the default of a class (instead of the enum reference) causes an unhelpful backtrace.

tbh, I'm not entirely sure what the fix here is, but based on how much effort I put into finding this mistake, I think there's room for some improvement in developer experience.

What I Did

This code:

class SiteFlavors(enum.Enum):
    SIMPLE = "simple"


@attrs.define
class Site():
    flavor: SiteFlavors = "simple"

attrs.unstructure(Site())

Causes this error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/astraluma/code/teahouse/barista/.venv/lib/python3.12/site-packages/cattrs/converters.py", line 238, in unstructure
    return self._unstructure_func.dispatch(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<cattrs generated unstructure __main__.Site>", line 3, in unstructure_Site
  File "/home/astraluma/code/teahouse/barista/.venv/lib/python3.12/site-packages/cattrs/converters.py", line 359, in _unstructure_enum
    return obj.value
           ^^^^^^^^^
AttributeError: 'str' object has no attribute 'value'

which is obvious when reduced, but in my case, the usage was spread out between a definition, usage, and a wrapping library, so it looked more like:

  File "/app/src/barista/routes/sites.py", line 44, in delete_site
    await sitedb.attempt_delete(site)
  File "/usr/local/lib/python3.13/site-packages/chaise/__init__.py", line 441, in attempt_delete
    _, db, docid, etag = self._doc2blob(doc)
                         ~~~~~~~~~~~~~~^^^^^
  File "/usr/local/lib/python3.13/site-packages/chaise/__init__.py", line 308, in _doc2blob
    blob = self._session.loader().dumpj(doc)
  File "/usr/local/lib/python3.13/site-packages/chaise/__init__.py", line 129, in dumpj
    blob = self.dump_doc(doc)
  File "/usr/local/lib/python3.13/site-packages/chaise/attrs.py", line 194, in dump_doc
    return converter.unstructure(doc)
           ~~~~~~~~~~~~~~~~~~~~~^^^^^
  File "/usr/local/lib/python3.13/site-packages/cattrs/converters.py", line 238, in unstructure
    return self._unstructure_func.dispatch(
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        obj.__class__ if unstructure_as is None else unstructure_as
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    )(obj)
    ~^^^^^
  File "<cattrs generated unstructure barista.models.Site>", line 4, in unstructure_Site
    'flavor': __c_unstr_flavor(instance.flavor),
              ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/cattrs/converters.py", line 359, in _unstructure_enum
    return obj.value
           ^^^^^^^^^
AttributeError: 'str' object has no attribute 'value'