Hooking to determine type

[jpsnyder]

• cattrs version: 1.4.0
• Python version: 3.7.5
• Operating System: Windows

Description

Is there a way to create a special hook that determines the class/type to use based on the results of a function. Similar to register_structure_hook_func(), but for the value?
This would help to support the common technique of including a _type in the dict to determine the correct class within a base class.

As well, it would allow one to modify the value before being structured.

class Element:
    """Some generic base class"""
    ...

    @classmethod
    def _get_class(cls, value: dict):
        try:
            class_name = value["_type"]
            del value["_type"]
        except KeyError:
            return cls
            
        for klass in Element.__subclasses__():
            if klass.__name__ == class_name:
                return klass
        return cls
   

cattr.register_structure_type_hook(Element._get_class, base=Element)

[Tinche]

Here's how you can do this now:

from attr import define

from cattr import GenConverter

c = GenConverter()


@define
class Base:
    a: int


@define
class SubOne(Base):
    b: int


@define
class SubTwo(Base):
    c: int


def unstructure_base(b):
    return {"_type": b.__class__.__name__, **c.unstructure_attrs_asdict(b)}


c.register_unstructure_hook(Base, unstructure_base)


print(c.unstructure(SubOne(1, 2)))


def structure_base(val, _):
    sub_name = val.pop("_type")
    for klass in Base.__subclasses__():
        if klass.__name__ == sub_name:
            return c.structure_attrs_fromdict(val, klass)
    return c.structure_attrs_fromdict(val, Base)


c.register_structure_hook(Base, structure_base)

print(c.structure(c.unstructure(SubOne(1, 2)), Base))
print(c.structure(c.unstructure(SubTwo(1, 2)), Base))
print(c.structure(c.unstructure(Base(1)), Base))

A thing to keep in mind: structure and unstructure hooks use functools.singledispatch under the hood, so a hook registered for Base will apply to SubOne and SubTwo, unless there are hooks for those classes registered explicitly.

[jpsnyder]

Ah thank you! The missing piece of the puzzle for me was the usage of unstructure_attrs_asdict() and structure_attrs_fromdict()
I was trying to use the regular unstructure() and structure() functions within the hook which caused recursion errors. I'm sorry if I missed that part of the documentation.

[Tinche]

@jpsnyder ok to close this?

[jpsnyder]

Yes