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

doc/source/whatsnew/v1.3.0.rst

@@ -766,6 +766,7 @@ Indexing
 
 - Bug in :meth:`Index.union` dropping duplicate ``Index`` values when ``Index`` was not monotonic or ``sort`` was set to ``False`` (:issue:`36289`, :issue:`31326`, :issue:`40862`)
 - Bug in :meth:`CategoricalIndex.get_indexer` failing to raise ``InvalidIndexError`` when non-unique (:issue:`38372`)
+- Bug in :meth:`Series.loc` raising ``ValueError`` when input was filtered with a boolean list and values to set were a list with lower dimension (:issue:`20438`)
 - Bug in inserting many new columns into a :class:`DataFrame` causing incorrect subsequent indexing behavior (:issue:`38380`)
 - Bug in :meth:`DataFrame.__setitem__` raising ``ValueError`` when setting multiple values to duplicate columns (:issue:`15695`)
 - Bug in :meth:`DataFrame.loc`, :meth:`Series.loc`, :meth:`DataFrame.__getitem__` and :meth:`Series.__getitem__` returning incorrect elements for non-monotonic :class:`DatetimeIndex` for string slices (:issue:`33146`)

pandas/core/indexers.py

@@ -166,6 +166,8 @@ def check_setitem_lengths(indexer, value, values) -> bool:
         if is_list_like(value):
             if len(indexer) != len(value) and values.ndim == 1:
                 # boolean with truth values == len of the value is ok too
+                if isinstance(indexer, list):
+                    indexer = np.array(indexer)
                 if not (
                     isinstance(indexer, np.ndarray)
                     and indexer.dtype == np.bool_

pandas/tests/indexing/test_iloc.py

@@ -1114,6 +1114,20 @@ def test_iloc_interval(self):
         expected = DataFrame({Interval(1, 2): [2, 3]})
         tm.assert_frame_equal(result, expected)
 
+    @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