Dataclass default_factory field treated as required in union disambiguation

Ouverte Adaptée aux débutants
  • Quelque chose ne va pas ?
#796 0 commentaires 0 réactions 0 personnes assignées Voir sur GitHub

Les mainteneurs répondent en général sous 1 jour

@sammist2 y travaille déjà.

Depuis le 17/10/2026.

  • #800 par @Eryx-Z — fermée sans fusion
  • #801 par @sammist2 — ouverte

Évaluation

Difficulté
2/5
Temps estimé
1-3 heures
Accessibilité débutants
72/100
Type d'issue
Bug
Clarté
Clairement spécifiée
Activité
Active
Stack technique
python
Domaine
data

Piste de recherche

Commencez par exécuter le script de reproduction de l'issue pour confirmer que l'union de dataclass renvoie D2 pour une charge utile sans tags. La vérification de la valeur par défaut se trouve dans create_default_dis_func dans src/cattrs/disambiguators.py, autour des lignes 154-157, et fields_dict ainsi que adapted_fields dans src/cattrs/_compat.py montrent pourquoi default_factory y est invisible. C'est terminé lorsque le cas des dataclass correspond au comportement d'attrs, avec un test de default_factory ajouté dans tests/test_disambiguators.py.

Rédigé par le modèle d'indexation à partir du texte de l'issue.

Description

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.

Langage dominant
Python
Étoiles
1.1k
Forks
189
Merge moyen
22 h 19 min
PR mergées (30 j)
8

Préparer son environnement

Par où commencer

  1. Lisez l'issue en entier, puis le guide de contribution du projet.
  2. Signalez en commentaire que vous la prenez — cela évite que deux personnes fassent le même travail.
  3. Forkez le dépôt et travaillez sur une branche.
  4. Ouvrez une pull request qui référence le numéro de l'issue.

Autres issues de python-attrs/cattrs

Toutes les issues de python-attrs/cattrs

Issues similaires

Plus d'issues Python

Recevez les nouvelles issues par e-mail

Un résumé court des issues GitHub adaptées aux débutants.