tomlkit: handle native date and datetime objects

Author: Tinche

HISTORY.md

@@ -23,6 +23,9 @@ Our backwards-compatibility policy can be found [here](https://github.com/python
   ([##699](https://github.com/python-attrs/cattrs/issues/699))
 - _cattrs_ now tracks performance using [codspeed](https://codspeed.io/python-attrs/cattrs).
   ([#703](https://github.com/python-attrs/cattrs/pull/703))
+- The {mod}`tomlkit <cattrs.preconf.tomlkit>` preconf converter now properly handles `date` and `datetime` objects when structuring.
+  ([#707](https://github.com/python-attrs/cattrs/issues/707))
+
 
 ## 25.3.0 (2025-10-07)
 

docs/preconf.md

@@ -202,4 +202,6 @@ Found at {mod}`cattrs.preconf.tomlkit`.
 
 Bytes are serialized as base 85 strings. Sets are serialized as lists, and deserialized back into sets.
 Tuples are serialized as lists, and deserialized back into tuples.
-_tomlkit_ only supports mappings with string keys so mappings will have their keys stringified before serialization, and destringified during deserialization. `date` s are serialized as ISO 8601 strings.
+_tomlkit_ only supports mappings with string keys so mappings will have their keys stringified before serialization, and destringified during deserialization.
+`date` s are serialized as ISO 8601 strings. `datetime` s are passed through to be unstructured by _tomlkit_ itself.
+

src/cattrs/preconf/tomlkit.py

@@ -68,9 +68,13 @@ def key_handler(k: bytes):
     # datetime inherits from date, so identity unstructure hook used
     # here to prevent the date unstructure hook running.
     converter.register_unstructure_hook(datetime, lambda v: v)
-    converter.register_structure_hook(datetime, validate_datetime)
+    converter.register_structure_hook(
+        datetime, lambda v, _: v if isinstance(v, datetime) else validate_datetime(v, _)
+    )
     converter.register_unstructure_hook(date, lambda v: v.isoformat())
-    converter.register_structure_hook(date, lambda v, _: date.fromisoformat(v))
+    converter.register_structure_hook(
+        date, lambda v, _: v if isinstance(v, date) else date.fromisoformat(v)
+    )
     configure_union_passthrough(
         Union[str, String, bool, int, Integer, float, Float], converter
     )

tests/preconf/test_tomlkit.py

@@ -0,0 +1,25 @@
+from datetime import date, datetime
+
+from attrs import define
+from tomlkit import loads
+
+from cattrs.preconf.tomlkit import make_converter
+
+
+@define
+class Event:
+    event_date: date
+    event_datetime: datetime
+
+
+def test_tomlkit_dates():
+    """Native date and datetime objects from tomlkit are properly handled."""
+    toml_input = """
+event_date = 2025-12-16
+event_datetime = 2025-12-16T20:00:00
+"""
+    converter = make_converter()
+    parsed = loads(toml_input)
+    structured = converter.structure(parsed, Event)
+    assert structured.event_date == date(2025, 12, 16)
+    assert structured.event_datetime == datetime(2025, 12, 16, 20, 0, 0)