• cattrs version: 1.1.2, master
• Python version: 3.9
• Operating System: Linux

Description

I was trying to use register_unstructure_hook to add a handle that does normal asdict style unstructuring, plus adds an extra field (a type tag). I found that it worked the first time, but subsequent execution fails.

Test case:

import attr
import cattr

converter = cattr.GenConverter()


@attr.s
class C:
    a: int = attr.ib(default=1)


def handler(obj):
    return {
        'type_tag': obj.__class__.__name__,
        **converter.unstructure_attrs_asdict(obj)
    }


converter.register_unstructure_hook(C, handler)

inst = C()

expected = {'type_tag': 'C', 'a': 1}

unstructured1 = converter.unstructure(inst)
unstructured2 = converter.unstructure(inst)

assert unstructured1 == expected, repr(unstructured1)
assert unstructured2 == unstructured1, repr(unstructured2)

You see it fails on the final assertion, not the first i.e. the second call behaves differently from the first, which was very surprising.

I found that I'm possibly "doing it wrong" - if I use cattr.global_converter.unstructure_attrs_asdict instead of converter.unstructure_attrs_asdict I get the intended behaviour. It's not clear to me if this is a bug or something that needs to be added to the documentation.