Boolean indexing inconsistency

doc/source/whatsnew/v3.0.0.rst

@@ -697,6 +697,7 @@ Indexing
 - Bug in :meth:`DataFrame.from_records` throwing a ``ValueError`` when passed an empty list in ``index`` (:issue:`58594`)
 - Bug in :meth:`DataFrame.loc` with inconsistent behavior of loc-set with 2 given indexes to Series (:issue:`59933`)
 - Bug in :meth:`MultiIndex.insert` when a new value inserted to a datetime-like level gets cast to ``NaT`` and fails indexing (:issue:`60388`)
+- Bug in :meth:`Series.iloc` with inconsistent behavior between ``__getitem__`` and ``__setitem__`` for Series boolean indexers (:issue:`60994`)
 - Bug in printing :attr:`Index.names` and :attr:`MultiIndex.levels` would not escape single quotes (:issue:`60190`)
 
 Missing

pandas/core/indexing.py

@@ -1637,6 +1637,22 @@ def _has_valid_setitem_indexer(self, indexer) -> bool:
                 "Consider using .loc with a DataFrame indexer for automatic alignment.",
             )
 
+        # Check for Series boolean indexer
+        if (
+            com.is_bool_indexer(indexer)
+            and hasattr(indexer, "index")
+            and isinstance(indexer.index, Index)
+        ):
+            if indexer.index.inferred_type == "integer":
+                raise NotImplementedError(
+                    "iLocation based boolean "
+                    "indexing on an integer type "
+                    "is not available"
+                )
+            raise ValueError(
+                "iLocation based boolean indexing cannot use an indexable as a mask"
+            )
+
         if not isinstance(indexer, tuple):
             indexer = _tuplify(self.ndim, indexer)
 

pandas/tests/extension/base/reduce.py

@@ -105,6 +105,11 @@ def test_reduce_series_numeric(self, data, all_numeric_reductions, skipna):
             msg = (
                 "[Cc]annot perform|Categorical is not ordered for operation|"
                 "does not support operation|"
+                "unsupported operand type|"
+                "NotImplemented|"
+                "not supported between instances|"
+                "NotImplementedType|"
+                "iLocation based boolean"
             )
 
             with pytest.raises(TypeError, match=msg):

pandas/tests/frame/indexing/test_getitem.py

@@ -407,6 +407,21 @@ def test_getitem_frozenset_unique_in_column(self):
         expected = Series([1], name=frozenset(["KEY"]))
         tm.assert_series_equal(result, expected)
 
+    def test_series_boolean_indexer_iloc_consistency(self):
+        # GH#60994 - Test consistency between __getitem__ and __setitem__ for Series
+        # boolean indexers
+        ser = Series([0, 1, 2])
+        mask = Series([True, False, False])
+
+        # __getitem__ should raise NotImplementedError
+        msg = "iLocation based boolean indexing on an integer type is not available"
+        with pytest.raises(NotImplementedError, match=msg):
+            ser.iloc[mask]
+
+        # __setitem__ should also raise NotImplementedError for consistency
+        with pytest.raises(NotImplementedError, match=msg):
+            ser.iloc[mask] = 10
+
 
 class TestGetitemSlice:
     def test_getitem_slice_float64(self, frame_or_series):