BUG: fix Series constructor for scalar and Categorical dtype

Categories and ordering of the resulting Series were not the same as those of the passed Categorical dtype

Author: cbertinato

doc/source/whatsnew/v0.23.0.txt

@@ -691,6 +691,7 @@ Categorical
 - Bug in :meth:`Index.astype` with a categorical dtype where the resultant index is not converted to a :class:`CategoricalIndex` for all types of index (:issue:`18630`)
 - Bug in :meth:`Series.astype` and ``Categorical.astype()`` where an existing categorical data does not get updated (:issue:`10696`, :issue:`18593`)
 - Bug in :class:`Index` constructor with ``dtype=CategoricalDtype(...)`` where ``categories`` and ``ordered`` are not maintained (issue:`19032`)
+- Bug in :class:`Series` constructor with scalar and ``dtype=CategoricalDtype(...)`` where ``categories`` and ``ordered`` are not maintained (issue:`19565`)
 
 Datetimelike
 ^^^^^^^^^^^^

pandas/core/dtypes/cast.py

@@ -1178,7 +1178,8 @@ def construct_1d_arraylike_from_scalar(value, length, dtype):
         subarr = DatetimeIndex([value] * length, dtype=dtype)
     elif is_categorical_dtype(dtype):
         from pandas import Categorical
-        subarr = Categorical([value] * length)
+        subarr = Categorical([value] * length, dtype.categories,
+                             ordered=dtype.ordered)
     else:
         if not isinstance(dtype, (np.dtype, type(np.dtype))):
             dtype = dtype.dtype

pandas/tests/dtypes/test_cast.py

@@ -22,7 +22,8 @@
     maybe_convert_string_to_object,
     maybe_convert_scalar,
     find_common_type,
-    construct_1d_object_array_from_listlike)
+    construct_1d_object_array_from_listlike,
+    construct_1d_arraylike_from_scalar)
 from pandas.core.dtypes.dtypes import (
     CategoricalDtype,
     DatetimeTZDtype,
@@ -422,3 +423,15 @@ def test_cast_1d_array(self, datum1, datum2):
     @pytest.mark.parametrize('val', [1, 2., None])
     def test_cast_1d_array_invalid_scalar(self, val):
         pytest.raises(TypeError, construct_1d_object_array_from_listlike, val)
+
+    def test_cast_1d_arraylike_from_scalar_categorical(self):
+        # GH 19565 - Categorical result from scalar did not maintain categories
+        # and ordering of the passed dtype
+        cats = ['a', 'b', 'c']
+        cat_type = CategoricalDtype(categories=cats, ordered=False)
+        expected = pd.Categorical(['a', 'a'], categories=cats)
+        result = construct_1d_arraylike_from_scalar('a', len(expected),
+                                                    cat_type)
+        tm.assert_categorical_equal(result, expected,
+                                    check_category_order=True,
+                                    check_dtype=True)