BUG: Bug in loc raised ValueError when setting value via boolean list

doc/source/whatsnew/v1.2.0.rst

@@ -476,6 +476,7 @@ Indexing
 - Bug in :meth:`DataFrame.loc` returned requested key plus missing values when ``loc`` was applied to single level from :class:`MultiIndex` (:issue:`27104`)
 - Bug in indexing on a :class:`Series` or :class:`DataFrame` with a :class:`CategoricalIndex` using a listlike indexer containing NA values (:issue:`37722`)
 - Bug in :meth:`DataFrame.xs` ignored ``droplevel=False`` for columns (:issue:`19056`)
+- Bug in :meth:`Series.loc` raised a ``ValueError`` when input was filtered with a boolean list and values to set were a list with lower dimension (:issue:`20438`)
 
 Missing
 ^^^^^^^

pandas/core/indexing.py

@@ -1521,6 +1521,8 @@ def _convert_to_indexer(self, key, axis: int, is_setter: bool = False):
 
     def _get_setitem_indexer(self, key):
         # GH#32257 Fall through to let numpy do validation
+        if isinstance(key, list) and com.is_bool_indexer(key):
+            return np.array(key)
         return key
 
     # -------------------------------------------------------------------

pandas/tests/indexing/test_iloc.py

@@ -801,6 +801,20 @@ def test_iloc_setitem_empty_frame_raises_with_3d_ndarray(self):
         with pytest.raises(ValueError, match=msg):
             obj.iloc[nd3] = 0
 
+    @pytest.mark.parametrize("indexing_func", [list, np.array])
+    @pytest.mark.parametrize("rhs_func", [list, np.array])
+    def test_loc_setitem_boolean_list(self, rhs_func, indexing_func):
+        # GH#20438 testing specifically list key, not arraylike
+        ser = Series([0, 1, 2])
+        ser.iloc[indexing_func([True, False, True])] = rhs_func([5, 10])
+        expected = Series([5, 1, 10])
+        tm.assert_series_equal(ser, expected)
+
+        df = DataFrame({"a": [0, 1, 2]})
+        df.iloc[indexing_func([True, False, True])] = rhs_func([[5], [10]])
+        expected = DataFrame({"a": [5, 1, 10]})
+        tm.assert_frame_equal(df, expected)
+
 
 class TestILocErrors:
     # NB: this test should work for _any_ Series we can pass as