TaggedDict multiple inheritance is failing Pydantic validation

[ketozhang]

Description of the problem

Using pydantic for deserialization, the ASDF serialized node of the YAML tree comes in as a asdf.tagged.TaggedDict. Pydantic is having trouble validating this as if it's a regular dictionary. I identified this being due to TaggedDict having multiple-inheritance to both UserDict and dict. I have not seen examples where this is the intended implementation for custom dicts.

Example of the problem

from collections import UserDict

from asdf.tagged import TaggedDict
from pydantic import BaseModel


class Bar(BaseModel):
    a: int


class Foo(BaseModel):
    bar: Bar


Foo.model_validate({"bar": {"a": 1}})
Foo.model_validate({"bar": TaggedDict({"a": 1})})

pydantic_core._pydantic_core.ValidationError: 1 validation error for Foo
bar.a
  Field required [type=missing, input_value={'a': 1}, input_type=TaggedDict]
    For further information visit https://errors.pydantic.dev/2.9/v/missing

Success on simplest single-inheritance.

class SimpleDict(UserDict): ...

Foo.model_validate({"bar": SimpleDict({"a": 1})})

Fails on simplest multi-inheritance

class MultiInheritanceDict(UserDict, dict): ...

Foo.model_validate({"bar": MultiInheritanceDict({"a": 1})})

pydantic_core._pydantic_core.ValidationError: 1 validation error for Foo
bar.a
  Field required [type=missing, input_value={'a': 1}, input_type=MultiInheritanceDict]
    For further information visit https://errors.pydantic.dev/2.9/v/missing

System information

asdf version: 3.5.0
python version: 3.12.7
operating system: OSX 14.6.1

[ketozhang]

I don't use TaggedDict directly with pydantic, so I don't suggest this needs to be fixed. This is likely relevant for pydantic to solve.

However, I do question the multiple inheritance here. A cursory glance, it's not typical to see UserDict inheriting dict.