Fields with init=False Don't Get Serialized about cattrs

[Tinche]

Howdy!

Yeah, like you've noticed, cattrs skips init=False fields by default.

This can be changed on a case-by-case basis by generating and registering an unstructure hook for a class.

from cattrs import Converter
from cattrs.gen import make_dict_unstructure_fn

def test_class_with_init_false():
    foo = ClassWithInitFalse(a=1, b="b")

    conv = cattrs.Converter()
    conv.register_unstructure_hook(
        ClassWithInitFalse,
        make_dict_unstructure_fn(
            ClassWithInitFalse, conv, _cattrs_include_init_false=True
        ),
    )

    serialized = conv.unstructure(foo)
    assert serialized == {"a": 1, "b": "b", "d": 2}

This can be applied wider (for example, to all attrs classes) using a hook factory that's similar to this. That's a little more complex but I can show you how if you need to.

I'm going to close this now to keep the issue list tidy, let me know if you have further questions!

[akefirad]

Thanks @Tinche for the reply.
Yes, I’d like to enable it for all classes in my app. (Out of curiosity; why is it not enabled by default for all?)
So I’d appreciate if you show how it can be done (or point me to docs if there’s any for this)
Thanks.

[Tinche]

Yes, I’d like to enable it for all classes in my app.

Instead of a single unstructure hook for ClassWithInitFalse, we're going to define an unstructure hook factory for all attrs classes. Here's the code, it looks somewhat similar to the single-class case:

from attrs import field, frozen, has

import cattrs
from cattrs.gen import make_dict_unstructure_fn


@frozen
class ClassWithInitFalse:
    a: int
    b: str
    d: int = field(init=False, default=2)


def test_class_with_init_false():
    foo = ClassWithInitFalse(a=1, b="b")

    conv = cattrs.Converter()
    conv.register_unstructure_hook_factory(
        has,
        lambda t: make_dict_unstructure_fn(t, conv, _cattrs_include_init_false=True),
    )

    serialized = conv.unstructure(foo)
    assert serialized == {"a": 1, "b": "b", "d": 2}

(Out of curiosity; why is it not enabled by default for all?)

I find folks usually use init=False attributes for computed attributes that get set in __attrs_post_init__.

point me to docs if there’s any for this

I guess we don't have exact docs for this, but you can look at https://catt.rs/en/latest/usage.html#using-factory-hooks for a larger example for customizing attrs handling using factory hooks.