fix: Fix literal slice in group by (pola-rs#17242)

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

@@ -378,9 +378,11 @@ impl<'a> AggregationContext<'a> {
 
     /// Update the group tuples
     pub(crate) fn with_groups(&mut self, groups: GroupsProxy) -> &mut Self {
-        // In case of new groups, a series always needs to be flattened
-        self.with_series(self.flat_naive().into_owned(), false, None)
-            .unwrap();
+        if let AggState::AggregatedList(_) = self.agg_state() {
+            // In case of new groups, a series always needs to be flattened
+            self.with_series(self.flat_naive().into_owned(), false, None)
+                .unwrap();
+        }
         self.groups = Cow::Owned(groups);
         // make sure that previous setting is not used
         self.update_groups = UpdateGroups::No;

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

@@ -1,5 +1,5 @@
 use polars_core::prelude::*;
-use polars_core::utils::{slice_offsets, CustomIterTools};
+use polars_core::utils::{slice_offsets, Container, CustomIterTools};
 use polars_core::POOL;
 use rayon::prelude::*;
 use AnyValue::Null;
@@ -106,13 +106,18 @@ impl PhysicalExpr for SliceExpr {
         let mut ac_length = results.pop().unwrap();
         let mut ac_offset = results.pop().unwrap();
 
-        let groups = ac.groups();
-
         use AggState::*;
         let groups = match (&ac_offset.state, &ac_length.state) {
             (Literal(offset), Literal(length)) => {
                 let (offset, length) = extract_args(offset, length, &self.expr)?;
 
+                if let Literal(s) = ac.agg_state() {
+                    let s1 = s.slice(offset, length);
+                    ac.with_literal(s1);
+                    return Ok(ac);
+                }
+                let groups = ac.groups();
+
                 match groups.as_ref() {
                     GroupsProxy::Idx(groups) => {
                         let groups = groups
@@ -134,6 +139,7 @@ impl PhysicalExpr for SliceExpr {
                 }
             },
             (Literal(offset), _) => {
+                let groups = ac.groups();
                 let offset = extract_offset(offset, &self.expr)?;
                 let length = ac_length.aggregated();
                 check_argument(&length, groups, "length", &self.expr)?;
@@ -168,6 +174,7 @@ impl PhysicalExpr for SliceExpr {
                 }
             },
             (_, Literal(length)) => {
+                let groups = ac.groups();
                 let length = extract_length(length, &self.expr)?;
                 let offset = ac_offset.aggregated();
                 check_argument(&offset, groups, "offset", &self.expr)?;
@@ -202,6 +209,7 @@ impl PhysicalExpr for SliceExpr {
                 }
             },
             _ => {
+                let groups = ac.groups();
                 let length = ac_length.aggregated();
                 let offset = ac_offset.aggregated();
                 check_argument(&length, groups, "length", &self.expr)?;

py-polars/tests/unit/operations/test_group_by.py

@@ -1130,3 +1130,12 @@ def sub_col_min(column: str, min_column: str) -> pl.Expr:
         pl.List(pl.Float64),
         pl.List(pl.Float64),
     ]
+
+
+def test_grouped_slice_literals() -> None:
+    assert pl.DataFrame({"idx": [1, 2, 3]}).group_by(True).agg(
+        x=pl.lit([1, 2]).slice(
+            -1, 1
+        ),  # slices a list of 1 element, so remains the same element
+        x2=pl.lit(pl.Series([1, 2])).slice(-1, 1),
+    ).to_dict(as_series=False) == {"literal": [True], "x": [[1, 2]], "x2": [2]}