Backport PR #48711 on branch 1.5.x (REGR: Regression in DataFrame.loc when setting df with all True indexer)

doc/source/whatsnew/v1.5.1.rst

@@ -14,6 +14,7 @@ including other versions of pandas.
 
 Fixed regressions
 ~~~~~~~~~~~~~~~~~
+- Fixed Regression in :meth:`DataFrame.loc` when setting values as a :class:`DataFrame` with all ``True`` indexer (:issue:`48701`)
 - Regression in :func:`.read_csv` causing an ``EmptyDataError`` when using an UTF-8 file handle that was already read from (:issue:`48646`)
 - Fixed performance regression in :func:`factorize` when ``na_sentinel`` is not ``None`` and ``sort=False`` (:issue:`48620`)
 -

pandas/core/indexing.py

@@ -1982,7 +1982,7 @@ def _setitem_single_column(self, loc: int, value, plane_indexer):
             and self.obj.shape[0] == value.shape[0]
             and not is_empty_indexer(pi)
         ):
-            if is_list_like(pi):
+            if is_list_like(pi) and not is_bool_dtype(pi):
                 value = value[np.argsort(pi)]
             else:
                 # in case of slice

pandas/tests/frame/indexing/test_indexing.py

@@ -1405,6 +1405,15 @@ def test_loc_named_tuple_for_midx(self):
         )
         tm.assert_frame_equal(result, expected)
 
+    @pytest.mark.parametrize("col", [{}, {"name": "a"}])
+    def test_loc_setitem_reordering_with_all_true_indexer(self, col):
+        # GH#48701
+        n = 17
+        df = DataFrame({**col, "x": range(n), "y": range(n)})
+        expected = df.copy()
+        df.loc[n * [True], ["x", "y"]] = df[["x", "y"]]
+        tm.assert_frame_equal(df, expected)
+
 
 class TestDataFrameIndexingUInt64:
     def test_setitem(self, uint64_frame):