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Dataclass default_factory field treated as required in union disambiguation

Abierto Apto para principiantes
  • ¿Algo no está bien?
#796 0 comentarios 0 reacciones 0 asignados Ver en GitHub

Los mantenedores suelen responder en 1 día

@Eryx-Z ya está trabajando en esto.

Desde el 16/10/2026.

  • #800 de @Eryx-Z — abierto
  • #801 de @sammist2 — abierto

Evaluación

Dificultad
2/5
Tiempo estimado
1-3 horas
Aptitud para principiantes
72/100
Tipo de issue
Error
Claridad
Bien especificado
Estado de actividad
Activo
Stack tecnológico
python
Área
data

Línea de trabajo

Empieza ejecutando el script de reproducción del issue para confirmar que la unión de dataclass devuelve D2 para un payload que no tiene tags. La comprobación de valores por defecto está en create_default_dis_func en src/cattrs/disambiguators.py, alrededor de las líneas 154-157, y fields_dict y adapted_fields en src/cattrs/_compat.py muestran por qué default_factory es invisible ahí. Está terminado cuando el caso de dataclass coincide con el comportamiento de attrs, con un test de default_factory añadido en tests/test_disambiguators.py.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

A dataclass field with default_factory is treated as a required field when cattrs builds the disambiguator for a union of dataclasses, so a payload that omits it is silently structured as the other class.

Minimal example

from dataclasses import dataclass, field
from typing import Union

from attrs import Factory, define

from cattrs import Converter


@dataclass
class D1:
    x: int
    tags: list = field(default_factory=list)


@dataclass
class D2:
    x: int


c = Converter(omit_if_default=True)
payload = c.unstructure(D1(1))
print("payload:", payload)
for union in (Union[D1, D2], Union[D2, D1]):
    print(f"structure(payload, {union}) observed:", repr(c.structure(payload, union)))
print("expected: D1(x=1, tags=[]) or a 'Could not disambiguate' TypeError, as for attrs below")


@define
class A1:
    x: int
    tags: list = Factory(list)


@define
class A2:
    x: int


c2 = Converter(omit_if_default=True)
try:
    print("attrs twin:", repr(c2.structure(c2.unstructure(A1(1)), Union[A1, A2])))
except TypeError as e:
    print("attrs twin raises:", type(e).__name__, e)

Observed output

payload: {'x': 1}
structure(payload, typing.Union[__main__.D1, __main__.D2]) observed: D2(x=1)
structure(payload, typing.Union[__main__.D2, __main__.D1]) observed: D2(x=1)
expected: D1(x=1, tags=[]) or a 'Could not disambiguate' TypeError, as for attrs below
attrs twin raises: TypeError Could not disambiguate between <class '__main__.A1'>, <class '__main__.A2'>: no unique non-default attributes

D1(1) round-trips to D2(x=1) in both union orders, with no error. The same classes written with attrs and Factory(list) raise when the union hook is built.

Expected behaviour

A field with default_factory should count as having a default, the same as an attrs Factory and a dataclass plain default. The dataclass union should then raise the same "Could not disambiguate" TypeError as the attrs version, instead of picking tags as the key and falling back to D2.

Versions

  • Python 3.12.3
  • cattrs 26.2.1 (26.1.0 gives the same output)
  • attrs 26.1.0
  • OS: Linux

Cause

In create_default_dis_func (src/cattrs/disambiguators.py, main at 73fa273), line 154 takes cl_fields = fields_dict(get_origin(cl) or cl) and line 157 treats a field as required when cl_fields[orig_name].default in (NOTHING, MISSING). For a dataclass, fields_dict (src/cattrs/_compat.py lines 131-135) returns the raw dataclasses.Field, whose .default is MISSING when only default_factory is set. adapted_fields (_compat.py lines 155-159) already turns default_factory into Factory, but the default check doesn't use it, so tags becomes the unique required field of D1, and a payload without it goes to D2.

Proposed fix

Check defaults on the adapted_fields view, which already maps default_factory to Factory, or also treat default_factory is not MISSING as a default at line 157. A dataclass default_factory case in tests/test_disambiguators.py would cover it; the existing dataclass tests there only use required fields.

I'm happy to open a PR with this fix and a test.

Lenguaje dominante
Python
Estrellas
1.1k
Forks
171
Merge medio
14 h 46 min
PR fusionados (30 d)
5

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