ensure weekday starts at 0 (#4220)

polars/polars-time/src/chunkedarray/datetime.rs

@@ -58,7 +58,13 @@ pub trait DatetimeMethods: AsDatetime {
     /// Extract weekday from underlying NaiveDateTime representation.
     /// Returns the weekday number where monday = 0 and sunday = 6
     fn weekday(&self) -> UInt32Chunked {
-        cast_and_apply(self.as_datetime(), temporal::weekday)
+        let ca = self.as_datetime();
+        let f = match ca.time_unit() {
+            TimeUnit::Nanoseconds => datetime_to_weekday_ns,
+            TimeUnit::Microseconds => datetime_to_weekday_us,
+            TimeUnit::Milliseconds => datetime_to_weekday_ms,
+        };
+        ca.apply_kernel_cast::<UInt32Type>(&f)
     }
 
     /// Returns the ISO week number starting from 1.

polars/polars-time/src/chunkedarray/kernels.rs

@@ -157,3 +157,29 @@ to_temporal_unit!(
     i64,
     ArrowDataType::UInt32
 );
+
+#[cfg(feature = "dtype-datetime")]
+to_temporal_unit!(
+    datetime_to_weekday_ns,
+    p_weekday,
+    timestamp_ns_to_datetime,
+    i64,
+    ArrowDataType::UInt32
+);
+
+#[cfg(feature = "dtype-datetime")]
+to_temporal_unit!(
+    datetime_to_weekday_ms,
+    p_weekday,
+    timestamp_ms_to_datetime,
+    i64,
+    ArrowDataType::UInt32
+);
+#[cfg(feature = "dtype-datetime")]
+to_temporal_unit!(
+    datetime_to_weekday_us,
+    p_weekday,
+    timestamp_us_to_datetime,
+    i64,
+    ArrowDataType::UInt32
+);

py-polars/tests/test_datelike.py

@@ -1252,3 +1252,13 @@ def test_supertype_timezones_4174() -> None:
     # test if this runs without error
     date_to_fill = df["dt_London"][0]
     df["dt_London"] = df["dt_London"].shift_and_fill(1, date_to_fill)
+
+
+def test_weekday() -> None:
+    # monday
+    s = pl.Series([datetime(2020, 1, 6)])
+
+    for tu in ["ns", "us", "ms"]:
+        assert s.dt.cast_time_unit(tu).dt.weekday()[0] == 0
+
+    assert s.cast(pl.Date).dt.weekday()[0] == 0