Dataclasses: fix kwonly arg recognition (#638)

Author: Tinche

HISTORY.md

@@ -1,5 +1,7 @@
 # History
+
 ```{currentmodule} cattrs
+
 ```
 
 This project adheres to [Calendar Versioning](https://calver.org/).
@@ -9,6 +11,11 @@ The third number is for emergencies when we need to start branches for older rel
 
 Our backwards-compatibility policy can be found [here](https://github.com/python-attrs/cattrs/blob/main/.github/SECURITY.md).
 
+## 24.1.3 (UNRELEASED)
+
+- Fix structuring of keyword-only dataclass fields when not using detailed validation.
+  ([#637](https://github.com/python-attrs/cattrs/issues/637) [#638](https://github.com/python-attrs/cattrs/pull/638))
+
 ## 24.1.2 (2024-09-22)
 
 - Fix {meth}`BaseConverter.register_structure_hook` and {meth}`BaseConverter.register_unstructure_hook` type hints.

src/cattrs/_compat.py

@@ -170,6 +170,7 @@ def adapted_fields(cl) -> List[Attribute]:
                 True,
                 type=type_hints.get(attr.name, attr.type),
                 alias=attr.name,
+                kw_only=getattr(attr, "kw_only", False),
             )
             for attr in attrs
         ]

tests/test_dataclasses.py

@@ -2,9 +2,12 @@
 from typing import List
 
 import attr
+import pytest
 
 from cattrs import BaseConverter
 
+from ._compat import is_py310_plus
+
 
 @dataclasses.dataclass
 class Foo:
@@ -41,3 +44,20 @@ def test_dataclasses(converter: BaseConverter):
 
     assert converter.unstructure(struct) == unstruct
     assert converter.structure(unstruct, Foo) == struct
+
+
+@pytest.mark.skipif(not is_py310_plus, reason="kwonly fields are Python 3.10+")
+def test_kw_only_propagation(converter: BaseConverter):
+    """KW-only args work.
+
+    Reproducer from https://github.com/python-attrs/cattrs/issues/637.
+    """
+
+    @dataclasses.dataclass
+    class PartialKeywords:
+        a1: str = "Default"
+        a2: str = dataclasses.field(kw_only=True)
+
+    assert converter.structure({"a2": "Value"}, PartialKeywords) == PartialKeywords(
+        a1="Default", a2="Value"
+    )