Union with subclasses

• cattrs version: 24.1.2
• Python version: 3.11
• Operating System: Linux/MacOSX

Description

I am not sure how to handle union with subclass.

What I Did

import attr
import cattr
from cattrs.strategies import configure_tagged_union, include_subclasses


@attr.s(frozen=True, auto_attribs=True)
class A:
    pass


@attr.s(frozen=True, auto_attribs=True)
class B:
    pass


@attr.s(frozen=True, auto_attribs=True)
class B2(B):
    b: int



@attr.s(frozen=True, auto_attribs=True)
class C:
    x: A | B


CONVERTER = cattr.Converter()
configure_tagged_union(A | B, CONVERTER)
union_strategy = partial(configure_tagged_union, tag_name="type_name")
include_subclasses(B, CONVERTER, union_strategy=union_strategy)

if __name__ == "__main__":
    instance = C(x=B2(b=1))
    r = CONVERTER.unstructure(instance, unstructure_as=C)
    print(r)
    print(CONVERTER.structure(r, C))

Getting error as

Traceback (most recent call last):
  File "..../IPython/core/interactiveshell.py", line 3577, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-1-8e14e71c7dee>", line 1, in <module>
    CONVERTER.unstructure(instance, unstructure_as=C)
  File ".../python3.11/site-packages/cattrs/converters.py", line 300, in unstructure
    return self._unstructure_func.dispatch(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<cattrs generated unstructure __main__.C>", line 3, in unstructure_C
    'x': __c_unstr_x(instance.x),
          ^^^^^^^^^^^^^^^^^^^^^^^
  File ".../python3.11/site-packages/cattrs/strategies/_unions.py", line 82, in unstructure_tagged_union
    res = _exact_cl_unstruct_hooks[val.__class__](val)
          ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
KeyError: <class '__main__.B2'>
^CTraceback (most recent call last):
  File ".../example.py", line 99, in <module>
    r = CONVERTER.unstruct

My understanding is that configure_tagged_union does not register all the hooks for subclasses. I wonder what is the right/best way to workaround it?

Reproduce the example with cattrs 24.1.2 and inspect cattrs/strategies/_unions.py, especially configure_tagged_union, unstructure_tagged_union, and include_subclasses. Check how hooks are registered for B and B2, then define completion as structuring and unstructuring the C instance without the reported KeyError and confirming the round trip behavior.