Subclass registration with generic decorator subclass fails

[Darkdragon84]

• cattrs version: 24.1.0
• Python version: 3.11.9
• Operating System: Ubuntu 22.04

Description

I am using the decorator pattern to generate a decorator class Decorated from an abstract base class Base.

from abc import abstractmethod
from attrs import define


@define
class Base:
    @abstractmethod
    def meth(self): ...


@define
class Decorated(Base):
    base: Base

    def meth(self):
        pass

Since Base has many more subclasses and is widely used as an annotation I want to register all subclasses of Base with my converter.

from cattrs import Converter
from cattrs.strategies import include_subclasses

converter = Converter()
include_subclasses(Base, converter)

This works fine if Decorated is not a generic class, but as soon as I make the Base typed field generic,

from typing import Generic, TypeVar

T = TypeVar("T", bound=Base)

@define
class Decorated(Base, Generic[T]):
    base: T

    def meth(self):
        pass

include_subclasses(Base, converter)

the subclass registration for Base fails with

Traceback (most recent call last):
  File "/home/valentin/python/pCloudPython/misc/cattrs_playground/decorator_pattern_subclass_registration.py", line 38, in <module>
    include_subclasses(Base, converter)
  File "/home/valentin/.pyenv/versions/3.11.9/envs/os11/lib/python3.11/site-packages/cattrs/strategies/_subclasses.py", line 75, in include_subclasses
    _include_subclasses_without_union_strategy(
  File "/home/valentin/.pyenv/versions/3.11.9/envs/os11/lib/python3.11/site-packages/cattrs/strategies/_subclasses.py", line 117, in _include_subclasses_without_union_strategy
    dis_fn = converter._get_dis_func(subclass_union, overrides=overrides)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/valentin/.pyenv/versions/3.11.9/envs/os11/lib/python3.11/site-packages/cattrs/converters.py", line 976, in _get_dis_func
    return create_default_dis_func(
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/valentin/.pyenv/versions/3.11.9/envs/os11/lib/python3.11/site-packages/cattrs/disambiguators.py", line 61, in create_default_dis_func
    overrides = [
                ^
  File "/home/valentin/.pyenv/versions/3.11.9/envs/os11/lib/python3.11/site-packages/cattrs/disambiguators.py", line 62, in <listcomp>
    getattr(converter.get_structure_hook(c), "overrides", {}) for c in classes
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/valentin/.pyenv/versions/3.11.9/envs/os11/lib/python3.11/site-packages/cattrs/converters.py", line 574, in get_structure_hook
    self._structure_func.dispatch(type)
  File "/home/valentin/.pyenv/versions/3.11.9/envs/os11/lib/python3.11/site-packages/cattrs/dispatch.py", line 134, in dispatch_without_caching
    res = self._function_dispatch.dispatch(typ)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/valentin/.pyenv/versions/3.11.9/envs/os11/lib/python3.11/site-packages/cattrs/dispatch.py", line 76, in dispatch
    return handler(typ)
           ^^^^^^^^^^^^
  File "/home/valentin/.pyenv/versions/3.11.9/envs/os11/lib/python3.11/site-packages/cattrs/converters.py", line 1290, in gen_structure_attrs_fromdict
    return make_dict_structure_fn(
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/valentin/.pyenv/versions/3.11.9/envs/os11/lib/python3.11/site-packages/cattrs/gen/__init__.py", line 772, in make_dict_structure_fn
    return make_dict_structure_fn_from_attrs(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/valentin/.pyenv/versions/3.11.9/envs/os11/lib/python3.11/site-packages/cattrs/gen/__init__.py", line 349, in make_dict_structure_fn_from_attrs
    raise StructureHandlerNotFoundError(
cattrs.errors.StructureHandlerNotFoundError: Missing type for generic argument T, specify it when structuring.

This happens at registration time, not at (un)structuring time.

Is this expected/intended? Can this be mitigated?

[Tinche]

Hello, sorry for the delay, I have a baby nowdays.

The problem is I don't think include_subclasses is a good fit for generic classes. cattrs can structure a Decorated[int] for you easily, but it can't really structure a Decorated[T] because it doesn't know what T is. (Even if I made provisions to treat it as Decorated[Any], the behavior wouldn't be all that useful.)

I don't really know how to improve the situation.

[Darkdragon84]

Hi! First of all congratulations!! Thanks for the reply and sorry for my late reply. I understand very well, I have two little kids too 😁

🤔 I probably don't understand the mechanics enough, but why is it not possible to just register the generic class with unknown T and sort of register a fully specified unstructure hook upon unstructuring a concrete fully specified type, e.g. Decorated[int]? The general Decorated case could be caught by the existing handler (which doesn't know T) yet, which in turn dispatches a concrete version for Decorated[int].

Is the problem, that subclass registration should be closed in itself, i.e. there should be nothing unknown and left undone once it's called?