From 233f9b385a0a56b047483a77f292f904c8f4b6eb Mon Sep 17 00:00:00 2001
From: Luke Manley <lukemanley@gmail.com>
Date: Fri, 31 Jan 2025 02:56:16 -0500
Subject: [PATCH] fix: Fix all-null list aggregations returning Null dtype
 (#20992)

---
 .../src/chunked_array/list/min_max.rs         | 10 +++++----
 .../src/chunked_array/list/sum_mean.rs        |  5 +++--
 .../operations/namespaces/list/test_list.py   | 22 +++++++++++++++++++
 3 files changed, 31 insertions(+), 6 deletions(-)

diff --git a/crates/polars-ops/src/chunked_array/list/min_max.rs b/crates/polars-ops/src/chunked_array/list/min_max.rs
index ee76b74e7579..5d795ecaca7e 100644
--- a/crates/polars-ops/src/chunked_array/list/min_max.rs
+++ b/crates/polars-ops/src/chunked_array/list/min_max.rs
@@ -89,7 +89,7 @@ pub(super) fn list_min_function(ca: &ListChunked) -> PolarsResult<Series> {
                     unsafe { out.into_series().from_physical_unchecked(dt) }
                 })
             },
-            _ => Ok(ca
+            dt => ca
                 .try_apply_amortized(|s| {
                     let s = s.as_ref();
                     let sc = s.min_reduce()?;
@@ -97,7 +97,8 @@ pub(super) fn list_min_function(ca: &ListChunked) -> PolarsResult<Series> {
                 })?
                 .explode()
                 .unwrap()
-                .into_series()),
+                .into_series()
+                .cast(dt),
         }
     }
 
@@ -199,7 +200,7 @@ pub(super) fn list_max_function(ca: &ListChunked) -> PolarsResult<Series> {
                     unsafe { out.into_series().from_physical_unchecked(dt) }
                 })
             },
-            _ => Ok(ca
+            dt => ca
                 .try_apply_amortized(|s| {
                     let s = s.as_ref();
                     let sc = s.max_reduce()?;
@@ -207,7 +208,8 @@ pub(super) fn list_max_function(ca: &ListChunked) -> PolarsResult<Series> {
                 })?
                 .explode()
                 .unwrap()
-                .into_series()),
+                .into_series()
+                .cast(dt),
         }
     }
 
diff --git a/crates/polars-ops/src/chunked_array/list/sum_mean.rs b/crates/polars-ops/src/chunked_array/list/sum_mean.rs
index fb26cb17872e..9eb0c85feb71 100644
--- a/crates/polars-ops/src/chunked_array/list/sum_mean.rs
+++ b/crates/polars-ops/src/chunked_array/list/sum_mean.rs
@@ -106,7 +106,7 @@ pub(super) fn sum_with_nulls(ca: &ListChunked, inner_dtype: &DataType) -> Polars
             out.into_series()
         },
         // slowest sum_as_series path
-        _ => ca
+        dt => ca
             .try_apply_amortized(|s| {
                 s.as_ref()
                     .sum_reduce()
@@ -114,7 +114,8 @@ pub(super) fn sum_with_nulls(ca: &ListChunked, inner_dtype: &DataType) -> Polars
             })?
             .explode()
             .unwrap()
-            .into_series(),
+            .into_series()
+            .cast(dt)?,
     };
     out.rename(ca.name().clone());
     Ok(out)
diff --git a/py-polars/tests/unit/operations/namespaces/list/test_list.py b/py-polars/tests/unit/operations/namespaces/list/test_list.py
index bed606e7c7ef..1ee41d52bea7 100644
--- a/py-polars/tests/unit/operations/namespaces/list/test_list.py
+++ b/py-polars/tests/unit/operations/namespaces/list/test_list.py
@@ -2,6 +2,7 @@
 
 import re
 from datetime import date, datetime
+from typing import TYPE_CHECKING
 
 import numpy as np
 import pytest
@@ -14,6 +15,9 @@
 )
 from polars.testing import assert_frame_equal, assert_series_equal
 
+if TYPE_CHECKING:
+    from polars._typing import PolarsDataType
+
 
 def test_list_arr_get() -> None:
     a = pl.Series("a", [[1, 2, 3], [4, 5], [6, 7, 8, 9]])
@@ -979,3 +983,21 @@ def test_list_eval_element_schema_19345() -> None:
         ),
         pl.DataFrame({"a": [[1]]}),
     )
+
+
+@pytest.mark.parametrize(
+    ("agg", "inner_dtype", "expected_dtype"),
+    [
+        ("sum", pl.Int8, pl.Int64),
+        ("max", pl.Int8, pl.Int8),
+        ("sum", pl.Duration("us"), pl.Duration("us")),
+        ("min", pl.Duration("ms"), pl.Duration("ms")),
+        ("min", pl.String, pl.String),
+        ("max", pl.String, pl.String),
+    ],
+)
+def test_list_agg_all_null(
+    agg: str, inner_dtype: PolarsDataType, expected_dtype: PolarsDataType
+) -> None:
+    s = pl.Series([None, None], dtype=pl.List(inner_dtype))
+    assert getattr(s.list, agg)().dtype == expected_dtype