BUG: Fixed 19497 - previously, renaming an index changed its type if …

doc/source/whatsnew/v0.23.0.txt

@@ -1364,6 +1364,7 @@ Reshaping
 - Improved error message for :func:`DataFrame.merge` when there is no common merge key (:issue:`19427`)
 - Bug in :func:`DataFrame.join` which does an ``outer`` instead of a ``left`` join when being called with multiple DataFrames and some have non-unique indices (:issue:`19624`)
 - :func:`Series.rename` now accepts ``axis`` as a kwarg (:issue:`18589`)
+- Bug in :func:`~DataFrame.rename` where an Index of same-length tuples was converted to a MultiIndex (:issue:`19497`)
 - Comparisons between :class:`Series` and :class:`Index` would return a ``Series`` with an incorrect name, ignoring the ``Index``'s name attribute (:issue:`19582`)
 - Bug in :func:`qcut` where datetime and timedelta data with ``NaT`` present raised a ``ValueError`` (:issue:`19768`)
 - Bug in :func:`DataFrame.iterrows`, which would infers strings not compliant to `ISO8601 <https://en.wikipedia.org/wiki/ISO_8601>`_ to datetimes (:issue:`19671`)

pandas/core/internals.py

@@ -5296,7 +5296,7 @@ def _transform_index(index, func, level=None):
         return MultiIndex.from_tuples(items, names=index.names)
     else:
         items = [func(x) for x in index]
-        return Index(items, name=index.name)
+        return Index(items, name=index.name, tupleize_cols=False)
 
 
 def _putmask_smart(v, m, n):

pandas/tests/frame/test_alter_axes.py

@@ -579,6 +579,17 @@ def test_rename_bug(self):
                              columns=['2001-01-01'])
         assert_frame_equal(df, expected)
 
+    def test_rename_bug2(self):
+        # GH 19497
+        # rename was changing Index to MultiIndex if Index contained tuples
+
+        df = DataFrame(data=np.arange(3), index=[(0, 0), (1, 1), (2, 2)],
+                       columns=["a"])
+        df = df.rename({(1, 1): (5, 4)}, axis="index")
+        expected = DataFrame(data=np.arange(3), index=[(0, 0), (5, 4), (2, 2)],
+                             columns=["a"])
+        assert_frame_equal(df, expected)
+
     def test_reorder_levels(self):
         index = MultiIndex(levels=[['bar'], ['one', 'two', 'three'], [0, 1]],
                            labels=[[0, 0, 0, 0, 0, 0],