Custom unstructure function is not called when a union type is wrapped in generic types multiple times

• cattrs version: 22.2.0
• Python version: 3.9
• Operating System: Ubuntu 20.04

Description

When you wrap a union type multiple times, e.g., Optional[list[Union[A, B]]], the custom unstructure function on the union type is not called. I believe this is related to https://github.com/python-attrs/cattrs/issues/129.

What I Did

import attrs
import cattrs

from typing import Optional, Union

c = cattrs.Converter()


@attrs.define
class A:
    a: int


@attrs.define
class B:
    a: int


@attrs.define
class C:
    f: Optional[list[Union[A, B]]]


c.register_unstructure_hook(
    Union[A, B],
    lambda o: {"_type": o.__class__.__name__, **c.unstructure(o)},
)
c.register_structure_hook(
    Union[A, B], lambda o, t: c.structure(o, A if o["_type"] == "A" else B)
)

inst = C([A(1), B(2)])
unstructured = c.unstructure(inst)
assert unstructured["f"][0]["_type"] == "A"
assert unstructured["f"][1]["_type"] == "B"

assert c.structure(unstructured, C) == inst

You should get the following error:

❯ python reproduce.py 
Traceback (most recent call last):
  File "/home/peter/programming/seagull/reproduce.py", line 34, in <module>
    assert unstructured["f"][0]["_type"] == "A"
KeyError: '_type'

Start by running the provided Python reproduction with cattrs 22.2.0 and Python 3.9, then trace how the converter handles the custom hooks for Optional[list[Union[A, B]]]. Done means the assertions for _type tags and round-trip structuring pass without the KeyError.