Unstructuring subclasses

• cattrs version: 22.2.0
• Python version: 3.8
• Operating System: macOS

Description

Hi @Tinche, I am currently working on several projects where I need to (un)structure objects of subclasses of a common base class. Digging through the documentation and also some issues here on Github, I eventually found your example how this can be done with the current version of cattrs. However, I think I have found two problems (bugs? unsupported features?) with the approach and would love to hear your opinion.

But before I go into the details, a quick question upfront: Is this going to remain the "recommended" approach to unstructure objects from a class hierarchy? I am asking because I also saw #312 which seems about to be merged but I am not entirely sure how/if it's going to replace your described method.

So here are now the two issues I noticed:

1. With the version described above, your example no longer seems to work. In order to get it running, one now needs to explicitly set slotted=False. I guess this is probably due to due the changes defaults (see also #267)? On the other hand, I wonder why your example then originally worked because it already uses define instead of attr.s?
2. Still, I struggled a lot to get it running in my own code until I finally found the problem: The approach crashes when using from __future__ import annotations and throws the following exception:

Traceback (most recent call last):
  File "/usr/local/Caskroom/mambaforge/base/envs/baybe/lib/python3.8/site-packages/cattrs/converters.py", line 461, in _structure_attribute
    return self._structure_func.dispatch(type_)(value, type_)
  File "/usr/local/Caskroom/mambaforge/base/envs/baybe/lib/python3.8/site-packages/cattrs/converters.py", line 377, in _structure_error
    raise StructureHandlerNotFoundError(msg, type_=cl)
cattrs.errors.StructureHandlerNotFoundError: Unsupported type: 'int'. Register a structure hook for it.

Is the second issue related to #215 or is this a bug?

Reproduce the subclass unstructuring example from issue #140 with cattrs 22.2.0, including slotted=False and from __future__ import annotations. Start by inspecting cattrs/converters.py, especially _structure_attribute, and compare the behavior with the changes discussed in issues #267, #312, and #215. Done means the supported approach and the cause or resolution of both reported failures are clear.