Backport PR #64386 on branch 3.0.x (BUG: fix sort_index AssertionError with RangeIndex and level parameter) (#64882)

Co-authored-by: Fadi Younes <37001921+repeating@users.noreply.github.com>

Author: meeseeksmachine

doc/source/whatsnew/v3.0.2.rst

@@ -21,6 +21,7 @@ Enhancements
 Fixed regressions
 ~~~~~~~~~~~~~~~~~
 - Fixed a regression in :func:`json_normalize` that caused top-level missing entries to raise a ``TypeError``. Missing entries are once again interpreted as empty records (:issue:`64188`)
+- Fixed regression in :meth:`DataFrame.sort_index` and :meth:`Series.sort_index` raising ``AssertionError`` when called with ``level=`` on a :class:`RangeIndex` (:issue:`64383`)
 - Fixed regression in :meth:`HDFStore.select` where the ``where`` clause on a datetime index silently returned empty results when the index had non-nanosecond resolution (:issue:`64310`)
 - Fixed regression in :meth:`Series.interpolate` where ``limit_direction="both"`` with ``limit`` greater than the Series length raised ``ValueError`` (:issue:`64322`)
 

pandas/core/indexes/range.py

@@ -772,8 +772,7 @@ def equals(self, other: object) -> bool:
             return self._range == other._range
         return super().equals(other)
 
-    # error: Signature of "sort_values" incompatible with supertype "Index"
-    @overload  # type: ignore[override]
+    @overload
     def sort_values(
         self,
         *,
@@ -791,7 +790,7 @@ def sort_values(
         ascending: bool = ...,
         na_position: NaPosition = ...,
         key: Callable | None = ...,
-    ) -> tuple[Self, np.ndarray | RangeIndex]: ...
+    ) -> tuple[Self, np.ndarray]: ...
 
     @overload
     def sort_values(
@@ -801,7 +800,7 @@ def sort_values(
         ascending: bool = ...,
         na_position: NaPosition = ...,
         key: Callable | None = ...,
-    ) -> Self | tuple[Self, np.ndarray | RangeIndex]: ...
+    ) -> Self | tuple[Self, np.ndarray]: ...
 
     def sort_values(
         self,
@@ -810,7 +809,7 @@ def sort_values(
         ascending: bool = True,
         na_position: NaPosition = "last",
         key: Callable | None = None,
-    ) -> Self | tuple[Self, np.ndarray | RangeIndex]:
+    ) -> Self | tuple[Self, np.ndarray]:
         if key is not None:
             return super().sort_values(
                 return_indexer=return_indexer,
@@ -831,10 +830,10 @@ def sort_values(
 
         if return_indexer:
             if inverse_indexer:
-                rng = range(len(self) - 1, -1, -1)
+                indexer = np.arange(len(self) - 1, -1, -1, dtype=np.intp)
             else:
-                rng = range(len(self))
-            return sorted_index, RangeIndex(rng)
+                indexer = np.arange(len(self), dtype=np.intp)
+            return sorted_index, indexer
         else:
             return sorted_index
 

pandas/tests/frame/methods/test_sort_index.py

@@ -815,6 +815,13 @@ def test_sort_index_ascending_tuple(self, ascending):
 
         tm.assert_frame_equal(result, expected)
 
+    def test_sort_index_level_on_range_index(self):
+        # GH#64383: sort_index with level= on a RangeIndex raised AssertionError
+        df = DataFrame({"a": [1, 2, 3]})
+        df.index.names = ["foo"]
+        result = df.sort_index(level="foo")
+        tm.assert_frame_equal(result, df)
+
 
 class TestDataFrameSortIndexKey:
     def test_sort_multi_index_key(self):