BUG: sort_index(1, ignore_index=True, ascending=False) (#57297)

Author: mroeschke

doc/source/whatsnew/v3.0.0.rst

@@ -245,9 +245,9 @@ Styler
 
 Other
 ^^^^^
+- Bug in :meth:`DataFrame.sort_index` when passing ``axis="columns"`` and ``ignore_index=True`` and ``ascending=False`` not returning a :class:`RangeIndex` columns (:issue:`57293`)
 - Bug in :meth:`DataFrame.where` where using a non-bool type array in the function would return a ``ValueError`` instead of a ``TypeError`` (:issue:`56330`)
 
-
 .. ***DO NOT USE THIS SECTION***
 
 -

pandas/core/generic.py

@@ -5102,7 +5102,10 @@ def sort_index(
                 result = self.copy(deep=None)
 
             if ignore_index:
-                result.index = default_index(len(self))
+                if axis == 1:
+                    result.columns = default_index(len(self.columns))
+                else:
+                    result.index = default_index(len(self))
             if inplace:
                 return None
             else:

pandas/tests/frame/methods/test_sort_index.py

@@ -1003,6 +1003,23 @@ def test_axis_columns_ignore_index():
     tm.assert_frame_equal(result, expected)
 
 
+def test_axis_columns_ignore_index_ascending_false():
+    # GH 57293
+    df = DataFrame(
+        {
+            "b": [1.0, 3.0, np.nan],
+            "a": [1, 4, 3],
+            1: ["a", "b", "c"],
+            "e": [3, 1, 4],
+            "d": [1, 2, 8],
+        }
+    ).set_index(["b", "a", 1])
+    result = df.sort_index(axis="columns", ignore_index=True, ascending=False)
+    expected = df.copy()
+    expected.columns = RangeIndex(2)
+    tm.assert_frame_equal(result, expected)
+
+
 def test_sort_index_stable_sort():
     # GH 57151
     df = DataFrame(