Only coerce to object if the calling index is not categorical

Author: jorisvandenbossche

pandas/indexes/base.py

@@ -1464,6 +1464,11 @@ def append(self, other):
         names = set([obj.name for obj in to_concat])
         name = None if len(names) > 1 else self.name
 
+        if self.is_categorical():
+            # if any of the to_concat is category
+            from pandas.indexes.category import CategoricalIndex
+            return CategoricalIndex._append_same_dtype(self, to_concat, name)
+
         typs = _concat.get_dtype_kinds(to_concat)
 
         if len(typs) == 1:

pandas/tests/indexes/test_category.py

@@ -272,12 +272,11 @@ def test_append(self):
 
         # with objects
         result = ci.append(Index(['c', 'a']))
-        #expected = CategoricalIndex(list('aabbcaca'), categories=categories)
-        expected = Index(list('aabbcaca'))
+        expected = CategoricalIndex(list('aabbcaca'), categories=categories)
         tm.assert_index_equal(result, expected, exact=True)
 
         # invalid objects
-        #self.assertRaises(TypeError, lambda: ci.append(Index(['a', 'd'])))
+        self.assertRaises(TypeError, lambda: ci.append(Index(['a', 'd'])))
 
         # GH14298 - if base object is not categorical -> coerce to object
         result = Index(['c', 'a']).append(ci)