BUG: Series.__getitem__ with CategoricalIndex[ints] listlike inconsistent with scalar (#45023)

Author: jbrockmendel

doc/source/whatsnew/v1.4.0.rst

@@ -717,6 +717,7 @@ Indexing
 - Bug in :meth:`IntervalIndex.get_indexer_non_unique` returning boolean mask instead of array of integers for a non unique and non monotonic index (:issue:`44084`)
 - Bug in :meth:`IntervalIndex.get_indexer_non_unique` not handling targets of ``dtype`` 'object' with NaNs correctly (:issue:`44482`)
 - Fixed regression where a single column ``np.matrix`` was no longer coerced to a 1d ``np.ndarray`` when added to a :class:`DataFrame` (:issue:`42376`)
+- Bug in :meth:`Series.__getitem__` with a :class:`CategoricalIndex` of integers treating lists of integers as positional indexers, inconsistent with the behavior with a single scalar integer (:issue:`15470`,:issue:`14865`)
 -
 
 Missing

pandas/core/indexes/category.py

@@ -16,7 +16,10 @@
     DtypeObj,
     npt,
 )
-from pandas.util._decorators import doc
+from pandas.util._decorators import (
+    cache_readonly,
+    doc,
+)
 from pandas.util._exceptions import find_stack_level
 
 from pandas.core.dtypes.common import (
@@ -180,6 +183,10 @@ class CategoricalIndex(NDArrayBackedExtensionIndex):
     def _can_hold_strings(self):
         return self.categories._can_hold_strings
 
+    @cache_readonly
+    def _should_fallback_to_positional(self) -> bool:
+        return self.categories._should_fallback_to_positional
+
     codes: np.ndarray
     categories: Index
     ordered: bool | None

pandas/tests/series/indexing/test_getitem.py

@@ -163,6 +163,27 @@ def test_getitem_scalar_categorical_index(self):
         result = ser[cats[0]]
         assert result == expected
 
+    def test_getitem_numeric_categorical_listlike_matches_scalar(self):
+        # GH#15470
+        ser = Series(["a", "b", "c"], index=pd.CategoricalIndex([2, 1, 0]))
+
+        # 0 is treated as a label
+        assert ser[0] == "c"
+
+        # the listlike analogue should also be treated as labels
+        res = ser[[0]]
+        expected = ser.iloc[-1:]
+        tm.assert_series_equal(res, expected)
+
+        res2 = ser[[0, 1, 2]]
+        tm.assert_series_equal(res2, ser.iloc[::-1])
+
+    def test_getitem_integer_categorical_not_positional(self):
+        # GH#14865
+        ser = Series(["a", "b", "c"], index=Index([1, 2, 3], dtype="category"))
+        assert ser.get(3) == "c"
+        assert ser[3] == "c"
+
     def test_getitem_str_with_timedeltaindex(self):
         rng = timedelta_range("1 day 10:11:12", freq="h", periods=500)
         ser = Series(np.arange(len(rng)), index=rng)