fix: Only slice after sort when slice is smaller than frame length (#…

…20084)

crates/polars-core/src/frame/mod.rs

@@ -2029,13 +2029,15 @@ impl DataFrame {
             return Ok(out);
         }
         if let Some((0, k)) = slice {
-            let desc = if sort_options.descending.len() == 1 {
-                sort_options.descending[0]
-            } else {
-                false
-            };
-            sort_options.limit = Some((k as IdxSize, desc));
-            return self.bottom_k_impl(k, by_column, sort_options);
+            if k < self.len() {
+                let desc = if sort_options.descending.len() == 1 {
+                    sort_options.descending[0]
+                } else {
+                    false
+                };
+                sort_options.limit = Some((k as IdxSize, desc));
+                return self.bottom_k_impl(k, by_column, sort_options);
+            }
         }
 
         #[cfg(feature = "dtype-struct")]

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

@@ -288,3 +288,14 @@ def test_slice_first_in_agg_18551() -> None:
         "x": ["B", "C"],
         "y": ["A", None],
     }
+
+
+def test_slice_after_sort_with_nulls_20079() -> None:
+    df = pl.LazyFrame({"a": [None, 1.2, None]})
+    out = df.sort("a", nulls_last=True).slice(0, 10).collect()
+    expected = pl.DataFrame({"a": [1.2, None, None]})
+    assert_frame_equal(out, expected)
+
+    out = df.sort("a", nulls_last=False).slice(0, 10).collect()
+    expected = pl.DataFrame({"a": [None, None, 1.2]})
+    assert_frame_equal(out, expected)