Make DateTime.astimezone timezone optional

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 ### Fixed
 
+- Allow `DateTime.astimezone()` to use the local timezone when no timezone argument is supplied, matching `datetime.datetime`.
+
 - Arithmetic and `replace()` results for `Integer`, `Float`, `String`, `DateTime`, `Date` and `Time` no longer share the original item's `Trivia`, so changing the result's comment or indentation no longer rewrites the item still stored in the document.
 
 ## [0.15.2] - 2026-10-16

tests/test_items.py

@@ -828,6 +828,18 @@ def test_datetimes_behave_like_datetimes(tz_utc: tzinfo, tz_pst: tzinfo) -> None
     assert doc.as_string() == "dt = 2018-07-23T12:34:56-05:00"
 
 
+def test_datetime_astimezone_defaults_to_local_timezone() -> None:
+    value = parse("time = 1979-05-27T07:32:00Z # birthday\n")["time"]
+    expected = datetime(1979, 5, 27, 7, 32, tzinfo=timezone.utc).astimezone()
+
+    result = value.astimezone()
+
+    assert isinstance(result, DateTime)
+    assert result.unwrap() == expected
+    assert result.as_string() == expected.isoformat()
+    assert result == value.astimezone(None)
+
+
 @pytest.mark.parametrize("fold", [0, 1])
 def test_datetime_subtraction_preserves_input_fold(tz_fold: tzinfo, fold: int) -> None:
     value = datetime(2018, 11, 4, 1, 30, tzinfo=tz_fold, fold=fold)

tomlkit/items.py

@@ -1184,7 +1184,7 @@ def __sub__(self, other: timedelta | datetime) -> DateTime | timedelta:
     def replace(self, *args: object, **kwargs: object) -> DateTime:
         return self._new(super().replace(*args, **kwargs))  # type: ignore[arg-type]
 
-    def astimezone(self, tz: tzinfo) -> DateTime:  # type: ignore[override]
+    def astimezone(self, tz: tzinfo | None = None) -> DateTime:
         result = super().astimezone(tz)
         if PY38:
             return result