From 41c7397b9b87b479ea272d0f679d7c6538ad367f Mon Sep 17 00:00:00 2001
From: Eryx-Z <257642323+Eryx-Z@users.noreply.github.com>
Date: Sat, 17 Oct 2026 04:43:50 +0800
Subject: [PATCH] fix: treat dataclass default_factory as a default in union
 disambiguation

create_default_dis_func used fields_dict, so a dataclass Field with only
default_factory kept .default as MISSING and was treated as required. That
made omit_if_default payloads with those fields silently pick the other
union member.

Use adapted_fields, which maps default_factory to Factory, matching attrs
Factory and plain dataclass defaults.

Fixes #796
---
 src/cattrs/disambiguators.py |  9 ++++++---
 tests/test_disambiguators.py | 17 +++++++++++++++++
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/src/cattrs/disambiguators.py b/src/cattrs/disambiguators.py
index 451d5768..62f5e483 100644
--- a/src/cattrs/disambiguators.py
+++ b/src/cattrs/disambiguators.py
@@ -14,7 +14,6 @@
 from ._compat import (
     NoneType,
     adapted_fields,
-    fields_dict,
     get_args,
     has,
     is_literal,
@@ -150,8 +149,12 @@ def dis_func(data: Mapping[Any, Any]) -> type | None:
             )
             uniq = cl_reqs - other_reqs
 
-            # We want a unique attribute with no default.
-            cl_fields = fields_dict(get_origin(cl) or cl)
+            # We want a unique attribute with no default. Use adapted_fields so
+            # dataclass default_factory is treated as a default (via Factory),
+            # matching attrs Factory and plain dataclass default.
+            cl_fields = {
+                a.name: a for a in adapted_fields(get_origin(cl) or cl)
+            }
             for maybe_renamed_attr_name in uniq:
                 orig_name = back_map[maybe_renamed_attr_name]
                 if cl_fields[orig_name].default in (NOTHING, MISSING):
diff --git a/tests/test_disambiguators.py b/tests/test_disambiguators.py
index 66f366d8..e39ae244 100644
--- a/tests/test_disambiguators.py
+++ b/tests/test_disambiguators.py
@@ -495,3 +495,20 @@ class B:
 
     assert converter.structure({"a": "a"}, Union[A, B]) == A()
     assert converter.structure({"b": "b"}, Union[A, B]) == B("b")
+
+
+def test_dataclass_default_factory_not_required():
+    """Dataclass default_factory fields count as defaults for disambiguation."""
+    import dataclasses
+
+    @dataclass
+    class D1:
+        x: int
+        tags: list = dataclasses.field(default_factory=list)
+
+    @dataclass
+    class D2:
+        x: int
+
+    with pytest.raises(TypeError, match="Could not disambiguate"):
+        create_default_dis_func(Converter(), D1, D2)