Lambda passed to register_unstructure_hook is not called for an instance when instance is in an inherited field

• cattrs version: 22.2.0
• Python version: 3.11
• Operating System: Windows 10

Description

I have a @frozen class that inherits a @frozen class containing two enums PType and Split. I am trying to write the class to a file using json.dumps. Unfortunately the unstructure_hook I provided is not getting called and as a result I get an Exception when dumping to json. I expect the PType unstructure_hook I provided to allow the class to be written to json.

What I Did

Here is the code that causes the exception. I have verified that the unstructure_hook I provided to c is never called.

    with open(MOVES_FILE_OUT, "w") as fo:
        c = cattrs.Converter()
        c.register_unstructure_hook(
            PType, lambda pt: pt.name
        )
        fo.write(json.dumps(c.unstructure(moves)))

This is the exception

TypeError: Object of type PType is not JSON serializable

My understanding is that the convertor calls the unstructure_hook when it sees a PType and coverts it to the type returned by the lambda (a str here), but it appears to miss the PType fields.

Start by reproducing the example with cattrs 22.2.0 on Python 3.11, including the inherited frozen classes and registered PType hook. Trace unstructuring of the inherited fields and verify that the PType hook is called and json.dumps accepts the resulting value.