fix(rust, python): fix all_null/sorted into_groups panic (#5733)

polars/polars-core/src/frame/groupby/into_groups.rs

@@ -83,6 +83,12 @@ where
         }
         let mut values = arr.values().as_slice();
         let null_count = arr.null_count();
+        let length = values.len();
+
+        // all nulls
+        if null_count == length {
+            return vec![[0, length as IdxSize]];
+        }
 
         let mut nulls_first = false;
         if null_count > 0 {
@@ -92,8 +98,8 @@ where
         if nulls_first {
             values = &values[null_count..];
         } else {
-            values = &values[..values.len() - null_count];
-        }
+            values = &values[..length - null_count];
+        };
 
         let n_threads = POOL.current_num_threads();
         let groups = if multithreaded && n_threads > 1 {

py-polars/tests/unit/test_groupby.py

@@ -220,3 +220,17 @@ def test_groupby_wildcard() -> None:
     assert df.groupby([pl.col("*")], maintain_order=True).agg(
         [pl.col("a").first().suffix("_agg")]
     ).to_dict(False) == {"a": [1, 2], "b": [1, 2], "a_agg": [1, 2]}
+
+
+def test_groupby_all_masked_out() -> None:
+    df = pl.DataFrame(
+        {
+            "val": pl.Series(
+                [None, None, None, None], dtype=pl.Categorical, nan_to_null=True
+            ).set_sorted(),
+            "col": [4, 4, 4, 4],
+        }
+    )
+    parts = df.partition_by("val")
+    assert len(parts) == 1
+    assert parts[0].frame_equal(df)