BUG: loc raising error for MultiIndex with bool indexer (#49766)

Author: phofl

doc/source/whatsnew/v2.0.0.rst

@@ -663,6 +663,7 @@ Indexing
 ^^^^^^^^
 - Bug in :meth:`DataFrame.reindex` filling with wrong values when indexing columns and index for ``uint`` dtypes (:issue:`48184`)
 - Bug in :meth:`DataFrame.loc` coercing dtypes when setting values with a list indexer (:issue:`49159`)
+- Bug in :meth:`DataFrame.loc` raising ``ValueError`` with ``bool`` indexer and :class:`MultiIndex` (:issue:`47687`)
 - Bug in :meth:`DataFrame.__setitem__` raising ``ValueError`` when right hand side is :class:`DataFrame` with :class:`MultiIndex` columns (:issue:`49121`)
 - Bug in :meth:`DataFrame.reindex` casting dtype to ``object`` when :class:`DataFrame` has single extension array column when re-indexing ``columns`` and ``index`` (:issue:`48190`)
 - Bug in :func:`~DataFrame.describe` when formatting percentiles in the resulting index showed more decimals than needed (:issue:`46362`)

pandas/core/indexing.py

@@ -1115,9 +1115,12 @@ def _validate_key(self, key, axis: Axis):
         # slice of labels (where start-end in labels)
         # slice of integers (only if in the labels)
         # boolean not in slice and with boolean index
+        ax = self.obj._get_axis(axis)
         if isinstance(key, bool) and not (
-            is_bool_dtype(self.obj._get_axis(axis))
-            or self.obj._get_axis(axis).dtype.name == "boolean"
+            is_bool_dtype(ax)
+            or ax.dtype.name == "boolean"
+            or isinstance(ax, MultiIndex)
+            and is_bool_dtype(ax.get_level_values(0))
         ):
             raise KeyError(
                 f"{key}: boolean label can not be used without a boolean index"

pandas/tests/frame/indexing/test_indexing.py

@@ -1436,6 +1436,24 @@ def test_loc_rhs_empty_warning(self):
             df.loc[:, "a"] = rhs
         tm.assert_frame_equal(df, expected)
 
+    @pytest.mark.parametrize("indexer", [True, (True,)])
+    @pytest.mark.parametrize("dtype", [bool, "boolean"])
+    def test_loc_bool_multiindex(self, dtype, indexer):
+        # GH#47687
+        midx = MultiIndex.from_arrays(
+            [
+                Series([True, True, False, False], dtype=dtype),
+                Series([True, False, True, False], dtype=dtype),
+            ],
+            names=["a", "b"],
+        )
+        df = DataFrame({"c": [1, 2, 3, 4]}, index=midx)
+        result = df.loc[indexer]
+        expected = DataFrame(
+            {"c": [1, 2]}, index=Index([True, False], name="b", dtype=dtype)
+        )
+        tm.assert_frame_equal(result, expected)
+
 
 class TestDataFrameIndexingUInt64:
     def test_setitem(self, uint64_frame):