BUG: Do not allow boolean values for indexing with .loc without a boolean index.

doc/source/whatsnew/v1.3.0.rst

@@ -642,6 +642,7 @@ Indexing
 - Bug in setting ``numpy.timedelta64`` values into an object-dtype :class:`Series` using a boolean indexer (:issue:`39488`)
 - Bug in setting numeric values into a into a boolean-dtypes :class:`Series` using ``at`` or ``iat`` failing to cast to object-dtype (:issue:`39582`)
 - Bug in :meth:`DataFrame.loc.__setitem__` when setting-with-expansion incorrectly raising when the index in the expanding axis contains duplicates (:issue:`40096`)
+- Bug in :meth:`DataFrame.loc` incorrectly allowing the lookup of boolean labels and slices (:issue:`20432`)
 
 Missing
 ^^^^^^^

pandas/core/dtypes/common.py

@@ -1386,7 +1386,7 @@ def is_bool_dtype(arr_or_dtype) -> bool:
         # we don't have a boolean Index class
         # so its object, we need to infer to
         # guess this
-        return arr_or_dtype.is_object and arr_or_dtype.inferred_type == "boolean"
+        return arr_or_dtype.is_object() and arr_or_dtype.inferred_type == "boolean"
     elif is_extension_array_dtype(arr_or_dtype):
         return getattr(dtype, "_is_boolean", False)
 

pandas/core/indexing.py

@@ -961,12 +961,17 @@ class _LocIndexer(_LocationIndexer):
 
     @doc(_LocationIndexer._validate_key)
     def _validate_key(self, key, axis: int):
-
         # valid for a collection of labels (we check their presence later)
         # slice of labels (where start-end in labels)
         # slice of integers (only if in the labels)
-        # boolean
-        pass
+        # boolean not in slice and with boolean index
+        if isinstance(key, bool) and not is_bool_dtype(self.obj.index):
+            raise KeyError("Boolean label can not be used without a boolean index")
+
+        if isinstance(key, slice) and (
+            isinstance(key.start, bool) or isinstance(key.stop, bool)
+        ):
+            raise TypeError("Boolean values can not be used in a slice")
 
     def _has_valid_setitem_indexer(self, indexer) -> bool:
         return True

pandas/tests/frame/indexing/test_indexing.py

@@ -947,10 +947,10 @@ def test_getitem_ix_boolean_duplicates_multiple(self):
     def test_getitem_setitem_ix_bool_keyerror(self):
         # #2199
         df = DataFrame({"a": [1, 2, 3]})
-
-        with pytest.raises(KeyError, match=r"^False$"):
+        message = "Boolean label can not be used without a boolean index"
+        with pytest.raises(KeyError, match=message):
             df.loc[False]
-        with pytest.raises(KeyError, match=r"^True$"):
+        with pytest.raises(KeyError, match=message):
             df.loc[True]
 
         msg = "cannot use a single bool to index into setitem"

pandas/tests/indexing/test_loc.py

@@ -2116,6 +2116,29 @@ def test_loc_getitem_slice_columns_mixed_dtype(self):
         tm.assert_frame_equal(df.loc[:, 1:], expected)
 
 
+class TestLocBooleanLabelsAndSlices(Base):
+    def test_loc_bool_incompatible_index_raises(self, index, frame_or_series):
+        # GH20432
+        message = "Boolean label can not be used without a boolean index"
+        if index.inferred_type != "boolean":
+            obj = frame_or_series(index=index, dtype="object")
+            with pytest.raises(KeyError, match=message):
+                obj.loc[True]
+
+    def test_loc_bool_should_not_raise(self, frame_or_series):
+        obj = frame_or_series(
+            index=Index([True, False], dtype="boolean"), dtype="object"
+        )
+        obj.loc[True]
+
+    def test_loc_bool_slice_raises(self, index, frame_or_series):
+        # GH20432
+        message = "Boolean values can not be used in a slice"
+        obj = frame_or_series(index=index, dtype="object")
+        with pytest.raises(TypeError, match=message):
+            obj.loc[True:False]
+
+
 class TestLocBooleanMask:
     def test_loc_setitem_bool_mask_timedeltaindex(self):
         # GH#14946