BUG: closes #764, set_index destroys column multiindex

Author: adamklein

pandas/core/index.py

@@ -1117,6 +1117,10 @@ def copy(self, order='C'):
     def dtype(self):
         return np.dtype('O')
 
+    @property
+    def _constructor(self):
+        return MultiIndex.from_tuples
+
     @staticmethod
     def _from_elements(values, labels=None, levels=None, names=None,
                        sortorder=None):

pandas/core/internals.py

@@ -599,7 +599,9 @@ def get_scalar(self, tup):
     def delete(self, item):
         i, _ = self._find_block(item)
         loc = self.items.get_loc(item)
-        new_items = Index(np.delete(np.asarray(self.items), loc))
+
+        new_items = self.items._constructor(
+                np.delete(np.asarray(self.items), loc))
 
         self._delete_from_block(i, item)
         self.set_items_norename(new_items)

pandas/tests/test_frame.py

@@ -2607,6 +2607,19 @@ def test_drop_duplicates(self):
         expected = df2.drop_duplicates(['A', 'B'], take_last=True)
         assert_frame_equal(result, expected)
 
+    def test_drop_col_still_multiindex(self):
+        arrays = [[  'a',   'b',   'c',    'top'],
+                  [  '',    '',    '',     'OD' ],
+                  [  '',    '',    '',     'wx' ]]
+
+        tuples = zip(*arrays)
+        tuples.sort()
+        index = MultiIndex.from_tuples(tuples)
+
+        df = DataFrame(randn(3,4), columns=index)
+        del df[('a','','')]
+        assert(isinstance(df.columns, MultiIndex))
+
     def test_fillna(self):
         self.tsframe['A'][:5] = nan
         self.tsframe['A'][-5:] = nan