Proof of concept for Copy-on-Write implementation

pandas/core/frame.py

@@ -3967,8 +3967,15 @@ def _set_value(
         """
         try:
             if takeable:
-                series = self._ixs(col, axis=1)
-                series._set_value(index, value, takeable=True)
+                if isinstance(self._mgr, ArrayManager):
+                    # with CoW, we can't use intermediate series
+                    # with takeable=True, we know that index is positional and
+                    # not a generic hashable label
+                    index = cast(int, index)
+                    self._mgr.column_setitem(col, index, value)
+                else:
+                    series = self._ixs(col, axis=1)
+                    series._set_value(index, value, takeable=True)
                 return
 
             series = self._get_item_cache(col)
@@ -4900,7 +4907,7 @@ def set_axis(self, labels, axis: Axis = 0, inplace: bool = False):
         "labels",
         [
             ("method", None),
-            ("copy", True),
+            ("copy", None),
             ("level", None),
             ("fill_value", np.nan),
             ("limit", None),
@@ -5084,7 +5091,7 @@ def rename(
         index: Renamer | None = None,
         columns: Renamer | None = None,
         axis: Axis | None = None,
-        copy: bool = True,
+        copy: bool | None = None,
         inplace: bool = False,
         level: Level | None = None,
         errors: str = "ignore",
@@ -5900,7 +5907,7 @@ class    max    type
         if inplace:
             new_obj = self
         else:
-            new_obj = self.copy()
+            new_obj = self.copy(deep=None)
 
         new_index = default_index(len(new_obj))
         if level is not None:

pandas/core/generic.py

@@ -996,7 +996,7 @@ def rename(
         index: Renamer | None = None,
         columns: Renamer | None = None,
         axis: Axis | None = None,
-        copy: bool_t = True,
+        copy: bool_t | None = None,
         inplace: bool_t = False,
         level: Level | None = None,
         errors: str = "ignore",
@@ -3952,6 +3952,8 @@ def _check_setitem_copy(self, t="setting", force=False):
         df.iloc[0:5]['group'] = 'a'
 
         """
+        if isinstance(self._mgr, (ArrayManager, SingleArrayManager)):
+            return
         # return early if the check is not needed
         if not (force or self._is_copy):
             return
@@ -4906,7 +4908,7 @@ def reindex(self: NDFrameT, *args, **kwargs) -> NDFrameT:
         axes, kwargs = self._construct_axes_from_arguments(args, kwargs)
         method = missing.clean_reindex_fill_method(kwargs.pop("method", None))
         level = kwargs.pop("level", None)
-        copy = kwargs.pop("copy", True)
+        copy = kwargs.pop("copy", None)
         limit = kwargs.pop("limit", None)
         tolerance = kwargs.pop("tolerance", None)
         fill_value = kwargs.pop("fill_value", None)
@@ -4931,9 +4933,7 @@ def reindex(self: NDFrameT, *args, **kwargs) -> NDFrameT:
             for axis, ax in axes.items()
             if ax is not None
         ):
-            if copy:
-                return self.copy()
-            return self
+            return self.copy(deep=copy)
 
         # check if we are a multi reindex
         if self._needs_reindex_multi(axes, method, level):
@@ -5895,7 +5895,7 @@ def astype(
         return cast(NDFrameT, result)
 
     @final
-    def copy(self: NDFrameT, deep: bool_t = True) -> NDFrameT:
+    def copy(self: NDFrameT, deep: bool_t | None = True) -> NDFrameT:
         """
         Make a copy of this object's indices and data.
 

pandas/core/indexing.py

@@ -1840,6 +1840,17 @@ def _setitem_single_column(self, loc: int, value, plane_indexer):
         """
         pi = plane_indexer
 
+        if not hasattr(self.obj._mgr, "blocks"):
+            # ArrayManager: in this case we cannot rely on getting the column
+            # as a Series to mutate, but need to operated on the mgr directly
+            if com.is_null_slice(pi) or com.is_full_slice(pi, len(self.obj)):
+                arr = self.obj._sanitize_column(value)
+                self.obj._mgr.iset(loc, arr)
+            else:
+                self.obj._mgr.column_setitem(loc, plane_indexer, value)
+            self.obj._clear_item_cache()
+            return
+
         ser = self.obj._ixs(loc, axis=1)
 
         # perform the equivalent of a setitem on the info axis