fix: Bug fix in existing fast path for sorted series (#20004)

crates/polars-core/src/chunked_array/ops/sort/mod.rs

@@ -138,7 +138,7 @@ macro_rules! sort_with_fast_path {
                 // if the nulls are already last we can clone
                 if $options.nulls_last && $ca.get($ca.len() - 1).is_none()  ||
                 // if the nulls are already first we can clone
-                $ca.get(0).is_none()
+                (!$options.nulls_last && $ca.get(0).is_none())
                 {
                     return $ca.clone();
                 }

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

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 from datetime import date, datetime
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, Callable
 
 import pytest
 from hypothesis import given
@@ -163,11 +163,30 @@ def test_sort_by_exprs() -> None:
     assert out.to_list() == [1, -1, 2, -2]
 
 
-def test_arg_sort_nulls() -> None:
+@pytest.mark.parametrize(
+    ("sort_function", "expected"),
+    [
+        (lambda x: x, ([0, 1, 2, 3, 4], [3, 4, 0, 1, 2])),
+        (
+            lambda x: x.sort(descending=False, nulls_last=True),
+            ([0, 1, 2, 3, 4], [3, 4, 0, 1, 2]),
+        ),
+        (
+            lambda x: x.sort(descending=False, nulls_last=False),
+            ([2, 3, 4, 0, 1], [0, 1, 2, 3, 4]),
+        ),
+    ],
+)
+def test_arg_sort_nulls(
+    sort_function: Callable[[pl.Series], pl.Series],
+    expected: tuple[list[int], list[int]],
+) -> None:
     a = pl.Series("a", [1.0, 2.0, 3.0, None, None])
 
-    assert a.arg_sort(nulls_last=True).to_list() == [0, 1, 2, 3, 4]
-    assert a.arg_sort(nulls_last=False).to_list() == [3, 4, 0, 1, 2]
+    a = sort_function(a)
+
+    assert a.arg_sort(nulls_last=True).to_list() == expected[0]
+    assert a.arg_sort(nulls_last=False).to_list() == expected[1]
 
     res = a.to_frame().sort(by="a", nulls_last=False).to_series().to_list()
     assert res == [None, None, 1.0, 2.0, 3.0]