Add whatsnew note and additional tests for rename_axis(inplace=True)

doc/source/whatsnew/v0.21.0.txt

@@ -33,6 +33,7 @@ Other Enhancements
 - The ``validate`` argument for :func:`merge` function now checks whether a merge is one-to-one, one-to-many, many-to-one, or many-to-many. If a merge is found to not be an example of specified merge type, an exception of type ``MergeError`` will be raised. For more, see :ref:`here <merging.validation>` (:issue:`16270`)
 - ``Series.to_dict()`` and ``DataFrame.to_dict()`` now support an ``into`` keyword which allows you to specify the ``collections.Mapping`` subclass that you would like returned.  The default is ``dict``, which is backwards compatible. (:issue:`16122`)
 - ``RangeIndex.append`` now returns a ``RangeIndex`` object when possible (:issue:`16212`)
+- ``Series.rename_axis()`` and ``DataFrame.rename_axis()`` with ``inplace=True`` now return None while renaming the axis inplace. (:issue:`15704`)
 - :func:`to_pickle` has gained a protocol parameter (:issue:`16252`). By default, this parameter is set to `HIGHEST_PROTOCOL <https://docs.python.org/3/library/pickle.html#data-stream-format>`__
 - :func:`api.types.infer_dtype` now infers decimals. (:issue: `15690`)
 - :func:`read_feather` has gained the ``nthreads`` parameter for multi-threaded operations (:issue:`16359`)

pandas/tests/frame/test_alter_axes.py

@@ -420,10 +420,19 @@ def test_rename(self):
 
     def test_rename_axis_inplace(self):
         # GH 15704
-        expected = self.frame.rename_axis('foo')
-        no_return = self.frame.rename_axis('foo', inplace=True)
+        frame = self.frame.copy()
+        expected = frame.rename_axis('foo')
+        result = frame.copy()
+        no_return = result.rename_axis('foo', inplace=True)
+
+        assert no_return is None
+        assert_frame_equal(result, expected)
+
+        expected = frame.rename_axis('bar', axis=1)
+        result = frame.copy()
+        no_return = result.rename_axis('bar', axis=1, inplace=True)
+
         assert no_return is None
-        result = self.frame
         assert_frame_equal(result, expected)
 
     def test_rename_multiindex(self):

pandas/tests/series/test_alter_axes.py

@@ -224,3 +224,13 @@ def test_reorder_levels(self):
 
         result = s.reorder_levels(['L0', 'L0', 'L0'])
         assert_series_equal(result, expected)
+
+    def test_rename_axis_inplace(self):
+        # GH 15704
+        series = self.ts.copy()
+        expected = series.rename_axis('foo')
+        result = series.copy()
+        no_return = result.rename_axis('foo', inplace=True)
+
+        assert no_return is None
+        assert_series_equal(result, expected)