BUG: Regression in .loc accepting a boolean Index as an indexer (#17738)

closes #17131

Author: jreback

doc/source/whatsnew/v0.21.0.txt

@@ -668,6 +668,7 @@ Indexing
 - Bug in ``IntervalIndex`` where performing a scalar lookup fails for included right endpoints of non-overlapping monotonic decreasing indexes (:issue:`16417`, :issue:`17271`)
 - Bug in :meth:`DataFrame.first_valid_index` and :meth:`DataFrame.last_valid_index` when no valid entry (:issue:`17400`)
 - Bug in :func:`Series.rename` when called with a `callable`, incorrectly alters the name of the `Series`, rather than the name of the `Index`. (:issue:`17407`)
+- Regression in ``.loc`` accepting a boolean ``Index`` as an indexer (:issue:`17131`)
 
 I/O
 ^^^

pandas/core/common.py

@@ -15,7 +15,7 @@
 from pandas import compat
 from pandas.compat import long, zip, iteritems
 from pandas.core.config import get_option
-from pandas.core.dtypes.generic import ABCSeries
+from pandas.core.dtypes.generic import ABCSeries, ABCIndex
 from pandas.core.dtypes.common import _NS_DTYPE
 from pandas.core.dtypes.inference import _iterable_not_string
 from pandas.core.dtypes.missing import isna, isnull, notnull  # noqa
@@ -182,7 +182,7 @@ def _maybe_box_datetimelike(value):
 
 
 def is_bool_indexer(key):
-    if isinstance(key, (ABCSeries, np.ndarray)):
+    if isinstance(key, (ABCSeries, np.ndarray, ABCIndex)):
         if key.dtype == np.object_:
             key = np.asarray(_values_from_object(key))
 

pandas/tests/indexing/test_loc.py

@@ -317,6 +317,23 @@ def test_loc_getitem_label_slice(self):
         self.check_result('mixed slice', 'loc', slice(2, 4, 2), 'ix', slice(
             2, 4, 2), typs=['mixed'], axes=0, fails=TypeError)
 
+    def test_loc_index(self):
+        # gh-17131
+        # a boolean index should index like a boolean numpy array
+
+        df = DataFrame(
+            np.random.random(size=(5, 10)),
+            index=["alpha_0", "alpha_1", "alpha_2", "beta_0", "beta_1"])
+
+        mask = df.index.map(lambda x: "alpha" in x)
+        expected = df.loc[np.array(mask)]
+
+        result = df.loc[mask]
+        tm.assert_frame_equal(result, expected)
+
+        result = df.loc[mask.values]
+        tm.assert_frame_equal(result, expected)
+
     def test_loc_general(self):
 
         df = DataFrame(