fix: Incorrect output type for map_groups returning all-NULL column (…

…#20743)

crates/polars-expr/src/expressions/apply.rs

@@ -300,6 +300,12 @@ impl ApplyExpr {
 
 fn all_unit_length(ca: &ListChunked) -> bool {
     assert_eq!(ca.chunks().len(), 1);
+
+    // Handles the Null dtype - in that case the offsets can be (e.g. [0,0,0 ...])
+    if ca.null_count() == ca.len() {
+        return true;
+    }
+
     let list_arr = ca.downcast_iter().next().unwrap();
     let offset = list_arr.offsets().as_slice();
     (offset[offset.len() - 1] as usize) == list_arr.len()

crates/polars-lazy/src/tests/aggregations.rs

@@ -576,3 +576,42 @@ fn test_take_in_groups() -> PolarsResult<()> {
     );
     Ok(())
 }
+
+#[test]
+fn test_anonymous_function_returns_scalar_all_null_20679() {
+    use std::sync::Arc;
+
+    fn reduction_function(column: Column) -> PolarsResult<Option<Column>> {
+        let val = column.get(0)?.into_static();
+        let col = Column::new_scalar("".into(), Scalar::new(column.dtype().clone(), val), 1);
+        Ok(Some(col))
+    }
+
+    let a = Column::new("a".into(), &[0, 0, 1]);
+    let dtype = DataType::Null;
+    let b = Column::new_scalar("b".into(), Scalar::new(dtype, AnyValue::Null), 3);
+    let df = DataFrame::new(vec![a, b]).unwrap();
+
+    let f = move |c: &mut [Column]| reduction_function(std::mem::take(&mut c[0]));
+
+    let expr = Expr::AnonymousFunction {
+        input: vec![col("b")],
+        function: LazySerde::Deserialized(SpecialEq::new(Arc::new(f))),
+        output_type: Default::default(),
+        options: FunctionOptions {
+            collect_groups: ApplyOptions::GroupWise,
+            fmt_str: "",
+            flags: FunctionFlags::default() | FunctionFlags::RETURNS_SCALAR,
+            ..Default::default()
+        },
+    };
+
+    let grouped_df = df
+        .lazy()
+        .group_by([col("a")])
+        .agg([expr])
+        .collect()
+        .unwrap();
+
+    assert_eq!(grouped_df.get_columns()[1].dtype(), &DataType::Null);
+}

py-polars/tests/unit/operations/map/test_map_groups.py

@@ -152,3 +152,16 @@ def test_map_groups_numpy_output_3057() -> None:
 
     expected = pl.DataFrame({"id": [0, 1], "result": [2.266666, 7.333333]})
     assert_frame_equal(result, expected)
+
+
+def test_map_groups_return_all_null_15260() -> None:
+    def foo(x: pl.Series) -> pl.Series:
+        return pl.Series([x[0][0]], dtype=x[0].dtype)
+
+    assert_frame_equal(
+        pl.DataFrame({"key": [0, 0, 1], "a": [None, None, None]})
+        .group_by("key")
+        .agg(pl.map_groups(exprs=["a"], function=foo))  # type: ignore[arg-type]
+        .sort("a"),
+        pl.DataFrame({"key": [0, 1], "a": [None, None]}),
+    )