REG: DataFrame.__setitem__(slice) is positional, closes pandas-dev#31469

Author: jbrockmendel

pandas/core/frame.py

@@ -2937,8 +2937,11 @@ def __setitem__(self, key, value):
             self._set_item(key, value)
 
     def _setitem_slice(self, key, value):
+        # NB: we can't just use self.loc[key] = value because that
+        #  operates on labels and we need to operate positional for
+        #  backwards-compat, xref GH#31469
         self._check_setitem_copy()
-        self.loc[key] = value
+        self.loc._setitem_with_indexer(key, value)
 
     def _setitem_array(self, key, value):
         # also raises Exception if object array with NA values

pandas/tests/frame/indexing/test_indexing.py

@@ -860,6 +860,15 @@ def test_fancy_getitem_slice_mixed(self, float_frame, float_string_frame):
 
         assert (float_frame["C"] == 4).all()
 
+    def test_setitem_slice_position(self):
+        # GH#31469
+        df = pd.DataFrame(np.zeros((100, 1)))
+        df[-4:] = 1
+        arr = np.zeros((100, 1))
+        arr[-4:] = 1
+        expected = pd.DataFrame(arr)
+        tm.assert_frame_equal(df, expected)
+
     def test_getitem_setitem_non_ix_labels(self):
         df = tm.makeTimeDataFrame()