feat: when calling convert_time_zone on time-zone-naive datetime, c…

…onvert as if from UTC

crates/polars-plan/src/dsl/function_expr/datetime.rs

@@ -329,16 +329,12 @@ pub(super) fn to_string(s: &Series, format: &str) -> PolarsResult<Series> {
 #[cfg(feature = "timezones")]
 pub(super) fn convert_time_zone(s: &Series, time_zone: &TimeZone) -> PolarsResult<Series> {
     match s.dtype() {
-        DataType::Datetime(_, Some(_)) => {
+        DataType::Datetime(_, _) => {
             let mut ca = s.datetime()?.clone();
             ca.set_time_zone(time_zone.clone())?;
             Ok(ca.into_series())
         },
-        _ => polars_bail!(
-            ComputeError:
-            "cannot call `convert_time_zone` on tz-naive; set a time zone first \
-            with `replace_time_zone`"
-        ),
+        dtype => polars_bail!(ComputeError: "expected Datetime, got {}", dtype),
     }
 }
 pub(super) fn with_time_unit(s: &Series, tu: TimeUnit) -> PolarsResult<Series> {

py-polars/polars/dataframe/frame.py

@@ -6395,7 +6395,7 @@ def join(
 
         Notes
         -----
-        For joining on columns with categorical data, see `pl.StringCache()`.
+        For joining on columns with categorical data, see :class:`pl.StringCache`.
         """
         if not isinstance(other, DataFrame):
             msg = f"expected `other` join table to be a DataFrame, got {type(other).__name__!r}"

py-polars/polars/expr/datetime.py

@@ -1548,6 +1548,11 @@ def convert_time_zone(self, time_zone: str) -> Expr:
         time_zone
             Time zone for the `Datetime` expression.
 
+        Notes
+        -----
+        If converting from a time-zone-naive datetime, then conversion will happen
+        as if converting from UTC, regardless of your system's time zone.
+
         Examples
         --------
         >>> from datetime import datetime

py-polars/polars/series/datetime.py

@@ -1092,6 +1092,11 @@ def convert_time_zone(self, time_zone: str) -> Series:
         time_zone
             Time zone for the `Datetime` Series.
 
+        Notes
+        -----
+        If converting from a time-zone-naive datetime, then conversion will happen
+        as if converting from UTC, regardless of your system's time zone.
+
         Examples
         --------
         >>> from datetime import datetime

py-polars/tests/unit/datatypes/test_temporal.py

@@ -1579,11 +1579,9 @@ def test_convert_time_zone_lazy_schema() -> None:
 
 def test_convert_time_zone_on_tz_naive() -> None:
     ts = pl.Series(["2020-01-01"]).str.strptime(pl.Datetime)
-    with pytest.raises(
-        ComputeError,
-        match="cannot call `convert_time_zone` on tz-naive; set a time zone first with `replace_time_zone`",
-    ):
-        ts.dt.convert_time_zone("Africa/Bamako")
+    result = ts.dt.convert_time_zone("Africa/Bamako").item()
+    expected = datetime(2020, 1, 1, 0, 0, tzinfo=ZoneInfo(key="Africa/Bamako"))
+    assert result == expected
 
 
 def test_tz_aware_get_idx_5010() -> None: