Allow add_categories() to accept Series/np.array and make raising on dup...

doc/source/whatsnew/v0.16.1.txt

@@ -55,6 +55,7 @@ Enhancements
 - Allow Panel.shift with ``axis='items'`` (:issue:`9890`)
 
 - Trying to write an excel file now raises ``NotImplementedError`` if the ``DataFrame`` has a ``MultiIndex`` instead of writing a broken Excel file. (:issue:`9794`)
+- Allow ``Categorical.add_categories`` to accept `Series` or `np.array`. (:issue:`9927`)
 
 .. _whatsnew_0161.api:
 

pandas/core/categorical.py

@@ -687,7 +687,7 @@ def add_categories(self, new_categories, inplace=False):
         if len(already_included) != 0:
             msg = "new categories must not include old categories: %s" % str(already_included)
             raise ValueError(msg)
-        new_categories = list(self._categories) + (new_categories)
+        new_categories = list(self._categories) + list(new_categories)
         new_categories = self._validate_categories(new_categories)
         cat = self if inplace else self.copy()
         cat._categories = new_categories

pandas/tests/test_categorical.py

@@ -727,6 +727,19 @@ def f():
             cat.add_categories(["d"])
         self.assertRaises(ValueError, f)
 
+        # GH 9927
+        cat = Categorical(list("abc"), ordered=True)
+        expected = Categorical(list("abc"), categories=list("abcde"), ordered=True)
+        # test with Series, np.array, index, list
+        res = cat.add_categories(Series(["d", "e"]))
+        self.assert_categorical_equal(res, expected)
+        res = cat.add_categories(np.array(["d", "e"]))
+        self.assert_categorical_equal(res, expected)
+        res = cat.add_categories(Index(["d", "e"]))
+        self.assert_categorical_equal(res, expected)
+        res = cat.add_categories(["d", "e"])
+        self.assert_categorical_equal(res, expected)
+
     def test_remove_categories(self):
         cat = Categorical(["a","b","c","a"], ordered=True)
         old = cat.copy()