BUG: disallow scalar in CategoricalIndex construtor (pandas-dev#38614)

Author: jbrockmendel

doc/source/whatsnew/v1.3.0.rst

@@ -171,7 +171,7 @@ Bug fixes
 
 Categorical
 ^^^^^^^^^^^
-
+- Bug in :class:`CategoricalIndex` incorrectly failing to raise ``TypeError`` when scalar data is passed (:issue:`38614`)
 - Bug in ``CategoricalIndex.reindex`` failed when ``Index`` passed with elements all in category (:issue:`28690`)
 - Bug where construcing a :class:`Categorical` from an object-dtype array of ``date`` objects did not round-trip correctly with ``astype`` (:issue:`38552`)
 

pandas/core/indexes/category.py

@@ -186,11 +186,7 @@ def __new__(
         name = maybe_extract_name(name, data, cls)
 
         if is_scalar(data):
-            # don't allow scalars
-            # if data is None, then categories must be provided
-            if data is not None or categories is None:
-                raise cls._scalar_data_error(data)
-            data = []
+            raise cls._scalar_data_error(data)
 
         data = Categorical(
             data, categories=categories, ordered=ordered, dtype=dtype, copy=copy

pandas/tests/indexes/categorical/test_category.py

@@ -91,7 +91,7 @@ def test_insert(self):
         tm.assert_index_equal(result, expected, exact=True)
 
         # test empty
-        result = CategoricalIndex(categories=categories).insert(0, "a")
+        result = CategoricalIndex([], categories=categories).insert(0, "a")
         expected = CategoricalIndex(["a"], categories=categories)
         tm.assert_index_equal(result, expected, exact=True)
 

pandas/tests/indexes/categorical/test_constructors.py

@@ -6,6 +6,11 @@
 
 
 class TestCategoricalIndexConstructors:
+    def test_construction_disallows_scalar(self):
+        msg = "must be called with a collection of some kind"
+        with pytest.raises(TypeError, match=msg):
+            CategoricalIndex(categories=list("abcd"), ordered=False)
+
     def test_construction(self):
 
         ci = CategoricalIndex(list("aabbca"), categories=list("abcd"), ordered=False)
@@ -20,7 +25,7 @@ def test_construction(self):
         assert not result.ordered
 
         # empty
-        result = CategoricalIndex(categories=categories)
+        result = CategoricalIndex([], categories=categories)
         tm.assert_index_equal(result.categories, Index(categories))
         tm.assert_numpy_array_equal(result.codes, np.array([], dtype="int8"))
         assert not result.ordered