TST: test pd.NaT with correct dtype

pandas/core/categorical.py

@@ -547,7 +547,7 @@ def _validate_categories(cls, categories, fastpath=False):
 
             # Categories cannot contain NaN.
             if categories.hasnans:
-                raise ValueError('Categorial categories cannot be NaN')
+                raise ValueError('Categorial categories cannot be null')
 
             # Categories must be unique.
             if not categories.is_unique:

pandas/tests/test_categorical.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 # pylint: disable=E1101,E1103,W0232
 
+import pytest
 import sys
 from datetime import datetime
 from distutils.version import LooseVersion
@@ -17,7 +18,8 @@
 import pandas.compat as compat
 import pandas.util.testing as tm
 from pandas import (Categorical, Index, Series, DataFrame, PeriodIndex,
-                    Timestamp, CategoricalIndex, isnull)
+                    Timestamp, CategoricalIndex, DatetimeIndex,
+                    isnull, NaT)
 from pandas.compat import range, lrange, u, PY3
 from pandas.core.config import option_context
 
@@ -223,15 +225,6 @@ def f():
         # vals = np.asarray(cat[cat.notnull()])
         # self.assertTrue(is_integer_dtype(vals))
 
-        # Cannot have NaN in categories
-        def f(null_value):
-            pd.Categorical([null_value, "a", "b", "c"],
-                           categories=[null_value, "a", "b", "c"])
-
-        self.assertRaises(ValueError, f, np.nan)
-        self.assertRaises(ValueError, f, pd.NaT)
-        self.assertRaises(ValueError, f, None)
-
         # corner cases
         cat = pd.Categorical([1])
         self.assertTrue(len(cat.categories) == 1)
@@ -281,6 +274,22 @@ def f(null_value):
             c = Categorical(np.array([], dtype='int64'),  # noqa
                             categories=[3, 2, 1], ordered=True)
 
+    def test_constructor_with_null(self):
+
+        # Cannot have NaN in categories
+        with pytest.raises(ValueError):
+            pd.Categorical([np.nan, "a", "b", "c"],
+                           categories=[np.nan, "a", "b", "c"])
+
+        with pytest.raises(ValueError):
+            pd.Categorical([None, "a", "b", "c"],
+                           categories=[None, "a", "b", "c"])
+
+        with pytest.raises(ValueError):
+            pd.Categorical(DatetimeIndex(['nat', '20160101']),
+                           categories=[NaT, Timestamp('20160101')])
+
+
     def test_constructor_with_index(self):
         ci = CategoricalIndex(list('aabbca'), categories=list('cab'))
         tm.assert_categorical_equal(ci.values, Categorical(ci))