Use subclasses of abstract outer class in nested class

Question

I want to use all subclasses of an abstract class in the nested config class of a pydantic class like this:

def custom_json_loads(classes, ....):
    ##use classes here for json parsing

class Outer(BaseModel, abc.ABC):
    name = "test"
    class Config:
        json_loads = partial(custom_json_loads, Outer.__subclasses__)

The aim of it all is that I know the OuterClass Type for my JSON and the name of the classes signify which instance of a subclass should be created

Eg I have BlueOuter, RedOuter, GreenOuter and in the json there would be "outer" : { "name" : "BlueOuter", ....}

But I don't want to import all possible variants of the subclasses because they evolve over time

Answer 1

Why not use a discriminated union ?

import abc
from typing import Annotated, Literal, Union

from pydantic import BaseModel, Field


class Outer(BaseModel, abc.ABC):
    ...


class BlueOuter(Outer):
    name: Literal["BlueOuter"]


class RedOuter(Outer):
    name: Literal["RedOuter"]


class GreenOuter(Outer):
    name: Literal["GreenOuter"]


OuterUnion = Annotated[
    Union[BlueOuter, RedOuter, GreenOuter], Field(discriminator="name")
]


class Foo(BaseModel):
    outer: OuterUnion


print(Foo.parse_raw('{"outer": {"name": "BlueOuter"}}'))
print(Foo.parse_raw('{"outer": {"name": "RedOuter"}}'))
print(Foo.parse_raw('{"outer": {"name": "GreenOuter"}}'))

Output:

outer=BlueOuter(name='BlueOuter')
outer=RedOuter(name='RedOuter')
outer=GreenOuter(name='GreenOuter')

If you worry about the need to mantain OuterUnion when a new Outer subclass is added, you could have a unit test to check that OuterUnion has all the subclasses of Outer :

class OrangeOuter(Outer):
    name: Literal["OrangeOuter"]


outer_union_classes = OuterUnion.__args__[0].__args__
for subclass in Outer.__subclasses__():
    assert (
        subclass in outer_union_classes
    ), f"{subclass.__name__} must be a member of OuterUnion (classes: {[c.__name__ for c in outer_union_classes]}). Please add it."

Output:

AssertionError: OrangeOuter must be a member of OuterUnion (classes: ['BlueOuter', 'RedOuter', 'GreenOuter']). Please add it.