fix: gather_every should work on agg context (pola-rs#13810)

py-polars/src/expr/general.rs

@@ -426,7 +426,7 @@ impl PyExpr {
     fn gather_every(&self, n: usize, offset: usize) -> Self {
         self.inner
             .clone()
-            .map(
+            .apply(
                 move |s: Series| {
                     polars_ensure!(n > 0, InvalidOperation: "gather_every(n): n can't be zero");
                     Ok(Some(s.gather_every(n, offset)))

py-polars/tests/unit/dataframe/test_df.py

@@ -392,6 +392,23 @@ def test_gather_every() -> None:
     assert_frame_equal(expected_df, df.gather_every(2, offset=1))
 
 
+def test_gather_every_agg() -> None:
+    df = pl.DataFrame(
+        {
+            "g": [1, 1, 1, 2, 2, 2],
+            "a": ["a", "b", "c", "d", "e", "f"],
+        }
+    )
+    out = df.group_by(pl.col("g")).agg(pl.col("a").gather_every(2)).sort("g")
+    expected = pl.DataFrame(
+        {
+            "g": [1, 2],
+            "a": [["a", "c"], ["d", "f"]],
+        }
+    )
+    assert_frame_equal(out, expected)
+
+
 def test_take_misc(fruits_cars: pl.DataFrame) -> None:
     df = fruits_cars
 

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

@@ -798,3 +798,10 @@ def test_list_get_logical_type() -> None:
         dtype=pl.Date,
     )
     assert_series_equal(out, expected)
+
+
+def test_list_eval_gater_every_13410() -> None:
+    df = pl.DataFrame({"a": [[1, 2, 3], [4, 5, 6]]})
+    out = df.with_columns(result=pl.col("a").list.eval(pl.element().gather_every(2)))
+    expected = pl.DataFrame({"a": [[1, 2, 3], [4, 5, 6]], "result": [[1, 3], [4, 6]]})
+    assert_frame_equal(out, expected)