Ordering of registered hooks affecting outcome of serialization

[ngchihuan]

Hi cattrs team,
I encountered a seemingly quite surprising feature of cattrs, namely that the ordering of registered hooks could affect the serialization outcome of nested attrs or dataclasses.
See the two cases below.

I guess it would be not too surprising if one understands that make_dict_unstructure_fn relies on context information (what has been registered and what is in namespace, perhaps).

But I think this could be confusing behavior, and more information could be added to the documentation. Or perhaps, some different mechanism should be implemented to make the ordering does not matter.

cattrs version: 24.1.2
python version: 3.9.6

from __future__ import annotations
from cattrs.preconf.orjson import make_converter
import attrs
from cattrs.gen import make_dict_unstructure_fn


@attrs.define
class AModel:
    a: int


@attrs.define
class TopModel:
    top: AModel


converter = make_converter()

obj = TopModel(top=AModel(1))
converter.register_unstructure_hook(AModel, lambda obj: "A")
converter.register_unstructure_hook(
    TopModel, make_dict_unstructure_fn(TopModel, converter)
)
se = converter.unstructure(obj, TopModel) #output :{'top': 'A'}

If the order of hooks is reversed

converter.register_unstructure_hook(
    TopModel, make_dict_unstructure_fn(TopModel, converter)
)
converter.register_unstructure_hook(AModel, lambda obj: "A")
se = converter.unstructure(obj, TopModel) #output: {'top': {'a': 1}}

[Tinche]

Whether the order of hook registration matters or not is technically dependent on which hooks you use, but the vast majority of default hooks uses early hook resolution, so in practice the order always matters.

I agree we should document it prominently. I'll add a section to the top of https://catt.rs/en/stable/customizing.html.

[ngchihuan]

@Tinche
Thanks a lot for the explanation, and I'm looking forward to the updated documentation.