Unstructuring subclassed built-ins

• cattrs version: 22.1.0
• Python version: 3.10.5
• Operating System: Linux

Description

Hi, I wanted to have fancy code and decided to subclass built-in list. I run into problems when I tried to unstructure it using cattrs. It behaves differently. I am not sure if this is on purpose or a bug. Can you please help me, should I register some hooks for all such classes? Thank you.

What I Did

from attr import define
from cattrs import GenConverter
from typing import TypeVar

T = TypeVar("T")


@define
class Foo:
    val: int


class mylist(list[T]):
    pass


def main() -> None:
    a = list[Foo]([Foo(val=1)])
    b = mylist[Foo]([Foo(val=1)])

    converter = GenConverter()

    print(converter.unstructure(a))  # [{'val': 1}]
    print(converter.unstructure(b))  # [Foo(val=1)]

Start with the provided Python reproducer and compare GenConverter.unstructure on list[Foo] and the subclassed mylist[Foo]. Trace how the converter dispatches these entry points and determine whether the differing output is intended. Done means the behavior is clarified and, if confirmed as a bug, covered by a regression test.