Block rounding durations to negative durations

crates/polars-time/src/round.rs

@@ -48,6 +48,9 @@ impl PolarsRound for DateChunked {
 #[cfg(feature = "dtype-duration")]
 impl PolarsRound for DurationChunked {
     fn round(&self, every: Duration, offset: Duration, _tz: Option<&Tz>) -> PolarsResult<Self> {
+        if every.negative {
+            polars_bail!(ComputeError: "cannot round a Duration to a negative duration")
+        }
         if !every.is_constant_duration() {
             polars_bail!(InvalidOperation: "Cannot round a Duration series to a non-constant duration.");
         }

crates/polars-time/src/truncate.rs

@@ -130,6 +130,9 @@ impl PolarsTruncate for DurationChunked {
         let out = if every.len() == 1 {
             if let Some(every) = every.get(0) {
                 let every_duration = Duration::parse(every);
+                if every_duration.negative {
+                    polars_bail!(ComputeError: "cannot truncate a Duration to a negative duration")
+                }
                 if every_duration.is_constant_duration() {
                     let every_units = to_time_unit(&every_duration);
 
@@ -152,6 +155,9 @@ impl PolarsTruncate for DurationChunked {
             try_binary_elementwise(self, every, |opt_duration, opt_every| {
                 if let (Some(duration), Some(every)) = (opt_duration, opt_every) {
                     let every_duration = Duration::parse(every);
+                    if every_duration.negative {
+                        polars_bail!(ComputeError: "cannot truncate a Duration to a negative duration")
+                    }
                     if every_duration.is_constant_duration() {
                         let every_units = to_time_unit(&every_duration);
 

py-polars/tests/unit/namespaces/test_datetime.py

@@ -531,7 +531,6 @@ def test_truncate_duration(time_unit: TimeUnit) -> None:
     ).dt.cast_time_unit(time_unit)
 
     assert_series_equal(durations.dt.truncate("10s"), expected)
-    assert_series_equal(durations.dt.truncate("-10s"), expected)
 
 
 def test_truncate_duration_zero() -> None:
@@ -591,7 +590,8 @@ def test_truncate_negative() -> None:
         {
             "date": [date(1895, 5, 7), date(1955, 11, 5)],
             "datetime": [datetime(1895, 5, 7), datetime(1955, 11, 5)],
-            "duration": ["-1m", "1m"],
+            "duration": [timedelta(minutes=1), timedelta(minutes=-1)],
+            "every": ["-1m", "1m"],
         }
     )
 
@@ -605,15 +605,25 @@ def test_truncate_negative() -> None:
     ):
         df.select(pl.col("datetime").dt.truncate("-1m"))
 
+    with pytest.raises(
+        ComputeError, match="cannot truncate a Duration to a negative duration"
+    ):
+        df.select(pl.col("duration").dt.truncate("-1m"))
+
     with pytest.raises(
         ComputeError, match="cannot truncate a Date to a negative duration"
     ):
-        df.select(pl.col("date").dt.truncate(pl.col("duration")))
+        df.select(pl.col("date").dt.truncate(pl.col("every")))
 
     with pytest.raises(
         ComputeError, match="cannot truncate a Datetime to a negative duration"
     ):
-        df.select(pl.col("datetime").dt.truncate(pl.col("duration")))
+        df.select(pl.col("datetime").dt.truncate(pl.col("every")))
+
+    with pytest.raises(
+        ComputeError, match="cannot truncate a Duration to a negative duration"
+    ):
+        df.select(pl.col("duration").dt.truncate(pl.col("every")))
 
 
 @pytest.mark.parametrize(
@@ -673,7 +683,6 @@ def test_round_duration(time_unit: TimeUnit) -> None:
     ).dt.cast_time_unit(time_unit)
 
     assert_series_equal(durations.dt.round("10s"), expected)
-    assert_series_equal(durations.dt.round("-10s"), expected)
 
 
 def test_round_duration_zero() -> None:
@@ -745,6 +754,11 @@ def test_round_negative() -> None:
     ):
         pl.Series([datetime(1895, 5, 7)]).dt.round("-1m")
 
+    with pytest.raises(
+        ComputeError, match="cannot round a Duration to a negative duration"
+    ):
+        pl.Series([timedelta(days=1)]).dt.round("-1m")
+
 
 @pytest.mark.parametrize(
     ("time_unit", "date_in_that_unit"),