BUG: Copy index(GH 13522) (pandas-dev#14005)

Author: conquistador1492

doc/source/whatsnew/v0.19.0.txt

@@ -907,6 +907,7 @@ Bug Fixes
 - Bug in ``io.json.json_normalize()``, where non-ascii keys raised an exception (:issue:`13213`)
 - Bug when passing a not-default-indexed ``Series`` as ``xerr`` or ``yerr`` in ``.plot()`` (:issue:`11858`)
 - Bug in area plot draws legend incorrectly if subplot is enabled or legend is moved after plot (matplotlib 1.5.0 is required to draw area plot legend properly) (issue:`9161`, :issue:`13544`)
+- Bug in ``DataFrame`` assignment with an object-dtyped ``Index`` where the resultant column is mutable to the original object. (:issue:`13522`)
 - Bug in matplotlib ``AutoDataFormatter``; this restores the second scaled formatting and re-adds micro-second scaled formatting (:issue:`13131`)
 - Bug in selection from a ``HDFStore`` with a fixed format and ``start`` and/or ``stop`` specified will now return the selected range (:issue:`8287`)
 - Bug in ``Series`` construction from a tuple of integers on windows not returning default dtype (int64) (:issue:`13646`)

pandas/core/frame.py

@@ -2638,6 +2638,8 @@ def reindexer(value):
                     value = com._asarray_tuplesafe(value)
             elif value.ndim == 2:
                 value = value.copy().T
+            elif isinstance(value, Index):
+                value = value.copy(deep=True)
             else:
                 value = value.copy()
 

pandas/tests/frame/test_mutate_columns.py

@@ -156,6 +156,13 @@ def test_insert(self):
         df.insert(0, 'baz', df['c'])
         self.assertEqual(df.columns.name, 'some_name')
 
+        # GH 13522
+        df = DataFrame(index=['A', 'B', 'C'])
+        df['X'] = df.index
+        df['X'] = ['x', 'y', 'z']
+        exp = DataFrame(data={'X': ['x', 'y', 'z']}, index=['A', 'B', 'C'])
+        assert_frame_equal(df, exp)
+
     def test_delitem(self):
         del self.frame['A']
         self.assertNotIn('A', self.frame)