Changed Warning to DeprecationWarning

Author: colevoncole

pandas/core/indexing.py

@@ -1962,7 +1962,7 @@ def _setitem_with_indexer_frame_value(self, indexer, value: DataFrame, name: str
 
                 self._setitem_single_column(loc, val, pi)
 
-    def _setitem_single_column(self, loc: int, value, plane_indexer) -> None:
+    def _setitem_single_column(self, loc: int, value, plane_indexer, is_iloc = False) -> None:
         """
 
         Parameters
@@ -2028,6 +2028,15 @@ def _setitem_single_column(self, loc: int, value, plane_indexer) -> None:
                 # warn either
                 pass
             else:
+                warnings.warn(
+                    "In a future version, `df.iloc[:, i] = newvals` will attempt "
+                    "to set the values inplace instead of always setting a new "
+                    "array. To retain the old behavior, use either "
+                    "`df[df.columns[i]] = newvals` or, if columns are non-unique, "
+                    "`df.isetitem(i, newvals)`",
+                    DeprecationWarning,
+                    stacklevel=find_stack_level(),
+                )
                 # TODO: how to get future behavior?
                 # TODO: what if we got here indirectly via loc?
                 pass

pandas/tests/indexing/test_loc.py

@@ -3221,9 +3221,9 @@ def test_getitem_loc_str_periodindex(self):
         series = Series(range(20), index=index)
         assert series.loc["2000-01-14"] == 9
 
-    def test_no_warnings_raised_loc(self):
+    def test_deprecation_warnings_raised_loc(self):
         # GH#48673
-        with tm.assert_produces_warning(None):
+        with tm.assert_produces_warning(DeprecationWarning):
             values = np.arange(4).reshape(2, 2)
             df = DataFrame(values, columns=["a", "b"])
             new = np.array([10, 11]).astype(np.int16)