fixup! ENH: Parametrized CategoricalDtype

Author: TomAugspurger

pandas/core/categorical.py

@@ -234,9 +234,12 @@ def __init__(self, values, categories=None, ordered=None, dtype=None,
                  fastpath=False):
 
         if dtype is not None:
-            if categories is not None or ordered is not None:
+            if isinstance(dtype, compat.string_types) and dtype == 'category':
+                dtype = CategoricalDtype(categories, ordered)
+            elif categories is not None or ordered is not None:
                 raise ValueError("Cannot specify both `dtype` and `categories`"
                                  " or `ordered`.")
+
             categories = dtype.categories
             ordered = dtype.ordered
 

pandas/tests/test_categorical.py

@@ -454,6 +454,11 @@ def test_constructor_dtype_and_others_raises(self):
         with tm.assert_raises_regex(ValueError, "Cannot"):
             Categorical(['a', 'b'], ordered=False, dtype=dtype)
 
+    def test_constructor_str_category(self):
+        result = Categorical(['a', 'b'], dtype='category')
+        expected = Categorical(['a', 'b'])
+        tm.assert_categorical_equal(result, expected)
+
     def test_from_codes(self):
 
         # too few categories