Reusing existing hooks

[AdrianSosic]

Hi @Tinche,

I already feel that the answer to my question is again going to be related to singledispatch vs predicates or similar 😄 but nevertheless I'm a bit lost at the moment. Would appreciate if you enlighten me once again 🤓:

In #632, we've clarified that machinery like make_dict_unstructure_fn reach into the passed converter and get already existing hooks for all the fields. But then why do I get the following behavior?

import cattrs
from attrs import define

converter = cattrs.Converter()


@define
class Pair:
    x: float
    y: float


@define(init=False)
class ContainerInner:
    pair: Pair

    def __init__(self, x: float, y: float):
        self.pair = Pair(x, y)


@define
class ContainerOuter:
    inner: ContainerInner


@cattrs.register_unstructure_hook
def _unstructure_container(container: ContainerInner) -> dict:
    return {"x": container.pair.x, "y": container.pair.y}


@cattrs.register_unstructure_hook
def _unstructure_container_outer(container: ContainerOuter) -> dict:
    fn = cattrs.gen.make_dict_unstructure_fn(ContainerOuter, converter)
    return fn(container)


container = ContainerOuter(ContainerInner(1.0, 2.0))
print(container)
dct = cattrs.unstructure(container)
print(dct)

This gives:

{'inner': {'pair': {'x': 1.0, 'y': 2.0}}}

meaning that the hook for ContainerInner is not used when assembling the hook for ConverterOuter using make_dict_unstructure_fn.

Can you tell me what's going on / where's the flaw in my reasoning?

[Tinche]

There are two converter objects at play here: the converter you're instantiating and passing into make_dict_unstructure_fn, and the global converter. When you call cattrs.register_unstructure_hook, you're changing the global converter.

You can either import the global converter (from cattrs import global_converter) and use it exclusively, or call register_unstructure_hook and unstructure on your own converter (probably better, less global state).

[AdrianSosic]

OK, I'm an idiot 🤦🏼 😅 I'm of course aware of the concept of using different converters – that's what happens when you keep staring at the completely wrong parts of the code 🙃 I was actually trying to compose one of the problems I noticed in my code base into this minimal example and only added the custom converter in the end, forgetting to actually use it in the code 😬

Anyway, thanks a lot for the quick reply. I'll close for now but might reopen once I actually manage to reproduce my problem in a minimal example, is that OK?

All the best, and as always, thanks for the great support!

[Tinche]

Sure, feel free to reopen or open a new one.