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`.
+
 - Fix valid TOML being rejected when an out-of-order table reopens an implicit super-table with a dotted key: `[a.b.c]` then `[a]` with `b.z = 1` extends the implicit `a.b` super-table rather than redefining it (stdlib `tomllib` accepts it), so it now parses and round-trips instead of raising `Redefinition of an existing table`. The concrete-table case (`[a.b]` then `[a]` `b.z = 1`) stays rejected. ([#565](https://github.com/python-poetry/tomlkit/pull/565))
 - Fix appending a key to a parsed inline table that uses `", "` separators emitting a bare `","` for the new entry, leaving the table inconsistently spaced (e.g. `{x = 1, y = 2,z = 3}`). ([#595](https://github.com/python-poetry/tomlkit/pull/595))
 

tests/test_items.py

@@ -803,6 +803,18 @@ 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

@@ -1181,7 +1181,7 @@ 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