fix(rust, python): handle concat_list with first lit value (#7235)

polars/polars-lazy/polars-plan/src/dsl/function_expr/list.rs

@@ -141,13 +141,22 @@ pub(super) fn concat(s: &mut [Series]) -> PolarsResult<Option<Series>> {
     let mut first = std::mem::take(&mut s[0]);
     let other = &s[1..];
 
-    let first_ca = match first.list().ok() {
+    let mut first_ca = match first.list().ok() {
         Some(ca) => ca,
         None => {
             first = first.reshape(&[-1, 1]).unwrap();
             first.list().unwrap()
         }
-    };
+    }
+    .clone();
+
+    if first_ca.len() == 1 && !other.is_empty() {
+        let max_len = other.iter().map(|s| s.len()).max().unwrap();
+        if max_len > 1 {
+            first_ca = first_ca.new_from_index(0, max_len)
+        }
+    }
+
     first_ca.lst_concat(other).map(|ca| Some(ca.into_series()))
 }
 

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

@@ -368,6 +368,18 @@ def test_flat_aggregation_to_list_conversion_6918() -> None:
     ).to_dict(False) == {"a": [1, 2], "b": [[[0.0, 1.0]], [[3.0, 4.0]]]}
 
 
+def test_concat_list_with_lit() -> None:
+    df = pl.DataFrame({"a": [1, 2, 3]})
+
+    assert df.select(pl.concat_list([pl.col("a"), pl.lit(1)]).alias("a")).to_dict(
+        False
+    ) == {"a": [[1, 1], [2, 1], [3, 1]]}
+
+    assert df.select(pl.concat_list([pl.lit(1), pl.col("a")]).alias("a")).to_dict(
+        False
+    ) == {"a": [[1, 1], [1, 2], [1, 3]]}
+
+
 def test_list_count_match() -> None:
     assert pl.DataFrame({"listcol": [[], [1], [1, 2, 3, 2], [1, 2, 1], [4, 4]]}).select(
         pl.col("listcol").arr.count_match(2).alias("number_of_twos")