BUG: Init categorical series from value

closes #12336
closes #12368

Author: rinoc

doc/source/whatsnew/v0.18.0.txt

@@ -1107,3 +1107,4 @@ Bug Fixes
 - Bug in ``crosstab`` where arguments with non-overlapping indexes would return a ``KeyError`` (:issue:`10291`)
 
 - Bug in ``DataFrame.apply`` in which reduction was not being prevented for cases in which ``dtype`` was not a numpy dtype (:issue:`12244`)
+- Bug when initializing categorical series with a scalar value. (:issue:`12336`)

pandas/core/series.py

@@ -2926,6 +2926,8 @@ def create_from_value(value, index, dtype):
 
         if is_datetimetz(dtype):
             subarr = DatetimeIndex([value] * len(index), dtype=dtype)
+        elif is_categorical_dtype(dtype):
+            subarr = Categorical([value] * len(index))
         else:
             if not isinstance(dtype, (np.dtype, type(np.dtype))):
                 dtype = dtype.dtype

pandas/tests/series/test_constructors.py

@@ -635,6 +635,14 @@ def test_fromValue(self):
         self.assertEqual(dates.dtype, 'M8[ns]')
         self.assertEqual(len(dates), len(self.ts))
 
+        # GH12336
+        # Test construction of categorical series from value
+        categorical = Series(0, index=self.ts.index, dtype="category")
+        expected = Series(0, index=self.ts.index).astype("category")
+        self.assertEqual(categorical.dtype, 'category')
+        self.assertEqual(len(categorical), len(self.ts))
+        tm.assert_series_equal(categorical, expected)
+
     def test_constructor_dtype_timedelta64(self):
 
         # basic