From bc6caa8792b4d2e39ef4a48063147faa5013dccf Mon Sep 17 00:00:00 2001
From: Weijie Guo <reswqa@163.com>
Date: Thu, 18 Jan 2024 20:34:33 +0800
Subject: [PATCH] fix: `gather_every` should work on agg context

---
 py-polars/src/expr/general.rs                |  2 +-
 py-polars/tests/unit/dataframe/test_df.py    | 17 +++++++++++++++++
 py-polars/tests/unit/namespaces/test_list.py |  7 +++++++
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/py-polars/src/expr/general.rs b/py-polars/src/expr/general.rs
index 88f7915a1fbc..1e166d3541cd 100644
--- a/py-polars/src/expr/general.rs
+++ b/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)))
diff --git a/py-polars/tests/unit/dataframe/test_df.py b/py-polars/tests/unit/dataframe/test_df.py
index 2b6d965734da..b559026a2ea2 100644
--- a/py-polars/tests/unit/dataframe/test_df.py
+++ b/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
 
diff --git a/py-polars/tests/unit/namespaces/test_list.py b/py-polars/tests/unit/namespaces/test_list.py
index 4abff393be14..cefea46ef69d 100644
--- a/py-polars/tests/unit/namespaces/test_list.py
+++ b/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)