Regression modifying obj with all false indexer (pandas-dev#40999)

Author: phofl

pandas/core/indexing.py

@@ -48,6 +48,7 @@
 from pandas.core.construction import array as pd_array
 from pandas.core.indexers import (
     check_array_indexer,
+    is_empty_indexer,
     is_exact_shape_match,
     is_list_like_indexer,
     length_of_indexer,
@@ -1882,7 +1883,11 @@ def _setitem_single_column(self, loc: int, value, plane_indexer):
         # GH#6149 (null slice), GH#10408 (full bounds)
         if com.is_null_slice(pi) or com.is_full_slice(pi, len(self.obj)):
             ser = value
-        elif is_array_like(value) and is_exact_shape_match(ser, value):
+        elif (
+            is_array_like(value)
+            and is_exact_shape_match(ser, value)
+            and not is_empty_indexer(pi, value)
+        ):
             if is_list_like(pi):
                 ser = value[np.argsort(pi)]
             else:

pandas/tests/frame/indexing/test_setitem.py

@@ -903,6 +903,14 @@ def test_setitem_boolean_indexing(self):
         with pytest.raises(ValueError, match="Item wrong length"):
             df1[df1.index[:-1] > 2] = -1
 
+    def test_loc_setitem_all_false_boolean_two_blocks(self):
+        # GH#40885
+        df = DataFrame({"a": [1, 2], "b": [3, 4], "c": "a"})
+        expected = df.copy()
+        indexer = Series([False, False], name="c")
+        df.loc[indexer, ["b"]] = DataFrame({"b": [5, 6]}, index=[0, 1])
+        tm.assert_frame_equal(df, expected)
+
 
 class TestDataFrameSetitemCopyViewSemantics:
     def test_setitem_always_copy(self, float_frame):