BUG: Fix sort_index(axis=1, ignore_index=True) (pandas-dev#56487)

* BUG: Fix sort_index(axis=1, ignore_index=True)

* Switch condition

doc/source/whatsnew/v2.2.0.rst

@@ -704,9 +704,10 @@ Other
 - Bug in :func:`infer_freq` and :meth:`DatetimeIndex.inferred_freq` with weekly frequencies and non-nanosecond resolutions (:issue:`55609`)
 - Bug in :meth:`DataFrame.apply` where passing ``raw=True`` ignored ``args`` passed to the applied function (:issue:`55009`)
 - Bug in :meth:`DataFrame.from_dict` which would always sort the rows of the created :class:`DataFrame`.  (:issue:`55683`)
+- Bug in :meth:`DataFrame.sort_index` when passing ``axis="columns"`` and ``ignore_index=True`` raising a ``ValueError`` (:issue:`56478`)
 - Bug in rendering ``inf`` values inside a a :class:`DataFrame` with the ``use_inf_as_na`` option enabled (:issue:`55483`)
 - Bug in rendering a :class:`Series` with a :class:`MultiIndex` when one of the index level's names is 0 not having that name displayed (:issue:`55415`)
-- Bug in the error message when assigning an empty dataframe to a column (:issue:`55956`)
+- Bug in the error message when assigning an empty :class:`DataFrame` to a column (:issue:`55956`)
 - Bug when time-like strings were being cast to :class:`ArrowDtype` with ``pyarrow.time64`` type (:issue:`56463`)
 
 .. ***DO NOT USE THIS SECTION***

pandas/core/generic.py

@@ -5321,11 +5321,11 @@ def sort_index(
         new_data = self._mgr.take(indexer, axis=baxis, verify=False)
 
         # reconstruct axis if needed
-        new_data.set_axis(baxis, new_data.axes[baxis]._sort_levels_monotonic())
-
-        if ignore_index:
-            axis = 1 if isinstance(self, ABCDataFrame) else 0
-            new_data.set_axis(axis, default_index(len(indexer)))
+        if not ignore_index:
+            new_axis = new_data.axes[baxis]._sort_levels_monotonic()
+        else:
+            new_axis = default_index(len(indexer))
+        new_data.set_axis(baxis, new_axis)
 
         result = self._constructor_from_mgr(new_data, axes=new_data.axes)
 

pandas/tests/frame/methods/test_sort_index.py

@@ -994,3 +994,11 @@ def test_sort_index_with_sliced_multiindex():
         ),
     )
     tm.assert_frame_equal(result, expected)
+
+
+def test_axis_columns_ignore_index():
+    # GH 56478
+    df = DataFrame([[1, 2]], columns=["d", "c"])
+    result = df.sort_index(axis="columns", ignore_index=True)
+    expected = DataFrame([[2, 1]])
+    tm.assert_frame_equal(result, expected)