Backport PR #37432 on branch 1.1.x

doc/source/whatsnew/v1.1.4.rst

@@ -27,6 +27,7 @@ Fixed regressions
 - Fixed regression where slicing :class:`DatetimeIndex` raised :exc:`AssertionError` on irregular time series with ``pd.NaT`` or on unsorted indices (:issue:`36953` and :issue:`35509`)
 - Fixed regression in certain offsets (:meth:`pd.offsets.Day() <pandas.tseries.offsets.Day>` and below) no longer being hashable (:issue:`37267`)
 - Fixed regression in :class:`StataReader` which required ``chunksize`` to be manually set when using an iterator to read a dataset (:issue:`37280`)
+- Fixed regression in setitem with :meth:`DataFrame.iloc` which raised error when trying to set a value while filtering with a boolean list (:issue:`36741`)
 - Fixed regression in :attr:`MultiIndex.is_monotonic_increasing` returning wrong results with ``NaN`` in at least one of the levels (:issue:`37220`)
 
 .. ---------------------------------------------------------------------------

pandas/core/indexing.py

@@ -1669,8 +1669,6 @@ def _setitem_with_indexer(self, indexer, value):
                         "length than the value"
                     )
 
-            pi = plane_indexer[0] if lplane_indexer == 1 else plane_indexer
-
             def isetter(loc, v):
                 # positional setting on column loc
                 ser = self.obj._ixs(loc, axis=1)
@@ -1680,15 +1678,15 @@ def isetter(loc, v):
                 # which means essentially reassign to the columns of a
                 # multi-dim object
                 # GH6149 (null slice), GH10408 (full bounds)
-                if isinstance(pi, tuple) and all(
+                if isinstance(plane_indexer, tuple) and all(
                     com.is_null_slice(idx) or com.is_full_slice(idx, len(self.obj))
-                    for idx in pi
+                    for idx in plane_indexer
                 ):
                     ser = v
                 else:
                     # set the item, possibly having a dtype change
                     ser = ser.copy()
-                    ser._mgr = ser._mgr.setitem(indexer=pi, value=v)
+                    ser._mgr = ser._mgr.setitem(indexer=plane_indexer, value=v)
                     ser._maybe_update_cacher(clear=True)
 
                 # reset the sliced object if unique

pandas/tests/frame/indexing/test_setitem.py

@@ -184,3 +184,12 @@ def test_setitem_extension_types(self, obj, dtype):
         df["obj"] = obj
 
         tm.assert_frame_equal(df, expected)
+
+    @pytest.mark.parametrize("klass", [list, np.array])
+    def test_iloc_setitem_bool_indexer(self, klass):
+        # GH: 36741
+        df = DataFrame({"flag": ["x", "y", "z"], "value": [1, 3, 4]})
+        indexer = klass([True, False, False])
+        df.iloc[indexer, 1] = df.iloc[indexer, 1] * 2
+        expected = DataFrame({"flag": ["x", "y", "z"], "value": [2, 3, 4]})
+        tm.assert_frame_equal(df, expected)