implement isetitem

pandas/core/frame.py

@@ -3642,6 +3642,29 @@ def _get_value(self, index, col, takeable: bool = False) -> Scalar:
         loc = engine.get_loc(index)
         return series._values[loc]
 
+    def isetitem(self, loc, value) -> None:
+        """
+        Set the given value in the column with position 'loc'.
+
+        This is a positional analogue to __setitem__.
+
+        Parameters
+        ----------
+        loc : int or sequence of ints
+        value : scalar or arraylike
+
+        Notes
+        -----
+        Unlike `frame.iloc[:, i] = value`, `frame.isetitem(loc, value)` will
+        _never_ try to set the values in place, but will always insert a new
+        array.
+
+        In cases where `frame.columns` is unique, this is equivalent to
+        `frame[frame.columns[i]] = value`.
+        """
+        arraylike = self._sanitize_column(value)
+        self._iset_item_mgr(loc, arraylike, inplace=False)
+
     def __setitem__(self, key, value):
         key = com.apply_if_callable(key, self)
 

pandas/core/indexing.py

@@ -2010,7 +2010,7 @@ def _setitem_single_column(self, loc: int, value, plane_indexer):
                     "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.iloc(axis=1)[i] = newvals`.",
+                    "`df.isetitem(i, newvals)`",
                     FutureWarning,
                     stacklevel=find_stack_level(),
                 )