BUG: fix sort_index with level keyword on single-level RangeIndex

doc/source/whatsnew/v3.1.0.rst

@@ -157,6 +157,7 @@ Interval
 Indexing
 ^^^^^^^^
 - Bugs in setitem-with-expansion when adding new rows failing to keep the original dtype in some cases (:issue:`32346`, :issue:`15231`, :issue:`47503`, :issue:`6485`, :issue:`25383`, :issue:`52235`, :issue:`17026`, :issue:`56010`)
+- Fixed bug in :meth:`DataFrame.sort_index` and :meth:`Series.sort_index` where passing the ``level`` keyword with a single-level :class:`RangeIndex` raised an ``AssertionError`` (:issue:`64383`)
 -
 
 Missing

pandas/core/sorting.py

@@ -97,6 +97,8 @@ def get_indexer_indexer(
             sort_remaining=sort_remaining,
             na_position=na_position,
         )
+        if isinstance(indexer, ABCRangeIndex):
+            indexer = indexer._data
     elif (np.all(ascending) and target.is_monotonic_increasing) or (
         not np.any(ascending) and target.is_monotonic_decreasing
     ):

pandas/tests/frame/methods/test_sort_index.py

@@ -1030,3 +1030,49 @@ def test_sort_index_stable_sort():
         columns=["dt", "value"],
     ).set_index(["dt"])
     tm.assert_frame_equal(result, expected)
+
+
+def test_sort_index_rangeindex_with_level_keyword():
+    # GH#64383 - sort_index with level keyword on single-level RangeIndex
+    # should not raise AssertionError
+    df = DataFrame({"a": [1, 2, 3], "foo": [0, 1, 2]}).set_index(["foo"], drop=True)
+    result = df.sort_index(level="foo")
+    expected = DataFrame({"a": [1, 2, 3]}, index=RangeIndex(3, name="foo"))
+    tm.assert_frame_equal(result, expected)
+
+    # Also test when the index is already a RangeIndex with a named index
+    df2 = DataFrame({"a": [1, 2, 3]})
+    df2.index.names = ["foo"]
+    result2 = df2.sort_index(level="foo")
+    expected2 = DataFrame({"a": [1, 2, 3]}, index=RangeIndex(3, name="foo"))
+    tm.assert_frame_equal(result2, expected2)
+
+    # Test unsorted RangeIndex
+    df3 = DataFrame({"a": [1, 2, 3], "foo": [1, 0, 2]}).set_index(["foo"], drop=True)
+    result3 = df3.sort_index(level="foo")
+    expected3 = DataFrame(
+        {"a": [2, 1, 3]},
+        index=pd.Index([0, 1, 2], name="foo"),
+    )
+    tm.assert_frame_equal(result3, expected3)
+
+    # Test reversed RangeIndex
+    df4 = DataFrame(
+        {"a": [1, 2, 3]}, index=RangeIndex(start=2, stop=-1, step=-1, name="foo")
+    )
+    result4 = df4.sort_index(level="foo")
+    expected4 = DataFrame(
+        {"a": [3, 2, 1]},
+        index=pd.Index([0, 1, 2], name="foo"),
+    )
+    tm.assert_frame_equal(result4, expected4)
+
+    # Test ascending=False
+    df5 = DataFrame({"a": [1, 2, 3]})
+    df5.index.names = ["foo"]
+    result5 = df5.sort_index(level="foo", ascending=False)
+    expected5 = DataFrame(
+        {"a": [3, 2, 1]},
+        index=pd.Index([2, 1, 0], name="foo"),
+    )
+    tm.assert_frame_equal(result5, expected5)