BUG: Series(Categorical(...)).unique() should not change the categories order

Author: toobaz

pandas/core/indexes/category.py

@@ -365,10 +365,7 @@ def unique(self, level=None):
         if level not in {0, self.name, None}:
             raise ValueError("Level {} not found".format(level))
         result = base.IndexOpsMixin.unique(self)
-        # CategoricalIndex._shallow_copy uses keeps original categories
-        # and ordered if not otherwise specified
-        return self._shallow_copy(result, categories=result.categories,
-                                  ordered=result.ordered)
+        return self._shallow_copy(result)
 
     @Appender(base._shared_docs['duplicated'] % _index_doc_kwargs)
     def duplicated(self, keep='first'):

pandas/tests/test_categorical.py

@@ -1708,15 +1708,17 @@ def test_unique_index_series(self):
         # if ordered=False
         exp = Categorical([3, 1, 2], categories=[3, 1, 2])
         tm.assert_categorical_equal(c.unique(), exp)
+        tm.assert_categorical_equal(pd.Series(c).unique(), exp)
 
+        exp = Categorical([3, 1, 2], categories=[3, 2, 1])
         tm.assert_index_equal(Index(c).unique(), Index(exp))
-        tm.assert_categorical_equal(pd.Series(c).unique(), exp)
 
         c = Categorical([1, 1, 2, 2], categories=[3, 2, 1])
         exp = Categorical([1, 2], categories=[1, 2])
         tm.assert_categorical_equal(c.unique(), exp)
-        tm.assert_index_equal(Index(c).unique(), Index(exp))
         tm.assert_categorical_equal(pd.Series(c).unique(), exp)
+        exp = Categorical([1, 2], categories=[3, 2, 1])
+        tm.assert_index_equal(Index(c).unique(), Index(exp))
 
         c = Categorical([3, 1, 2, 2, 1], categories=[3, 2, 1], ordered=True)
         # Categorical.unique keeps categories order if ordered=True