CLN: Cleanup after CoW setitem PRs (#53142)

Author: phofl

doc/source/whatsnew/v2.1.0.rst

@@ -14,10 +14,12 @@ including other versions of pandas.
 Enhancements
 ~~~~~~~~~~~~
 
-.. _whatsnew_210.enhancements.enhancement1:
+.. _whatsnew_210.enhancements.cow:
 
-enhancement1
-^^^^^^^^^^^^
+Copy-on-Write improvements
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+- Setting a :class:`Series` into a :class:`DataFrame` now creates a lazy instead of a deep copy (:issue:`53142`)
 
 .. _whatsnew_210.enhancements.enhancement2:
 

pandas/core/frame.py

@@ -3915,7 +3915,6 @@ def isetitem(self, loc, value) -> None:
  In cases where ``frame.columns`` is unique, this is equivalent to
  ``frame[frame.columns[i]] = value``.
  """
- using_cow = using_copy_on_write()
  if isinstance(value, DataFrame):
  if is_integer(loc):
  loc = [loc]
@@ -3927,13 +3926,11 @@ def isetitem(self, loc, value) -> None:
  )
 
  for i, idx in enumerate(loc):
- arraylike, refs = self._sanitize_column(
- value.iloc[:, i], using_cow=using_cow
- )
+ arraylike, refs = self._sanitize_column(value.iloc[:, i])
  self._iset_item_mgr(idx, arraylike, inplace=False, refs=refs)
  return
 
- arraylike, refs = self._sanitize_column(value, using_cow=using_cow)
+ arraylike, refs = self._sanitize_column(value)
  self._iset_item_mgr(loc, arraylike, inplace=False, refs=refs)
 
  def __setitem__(self, key, value):
@@ -4170,7 +4167,7 @@ def _set_item(self, key, value) -> None:
  Series/TimeSeries will be conformed to the DataFrames index to
  ensure homogeneity.
  """
- value, refs = self._sanitize_column(value, using_cow=using_copy_on_write())
+ value, refs = self._sanitize_column(value)
 
  if (
  key in self.columns
@@ -4813,7 +4810,7 @@ def insert(
  elif isinstance(value, DataFrame):
  value = value.iloc[:, 0]
 
- value, refs = self._sanitize_column(value, using_cow=using_copy_on_write())
+ value, refs = self._sanitize_column(value)
  self._mgr.insert(loc, column, value, refs=refs)
 
  def assign(self, **kwargs) -> DataFrame:
@@ -4884,9 +4881,7 @@ def assign(self, **kwargs) -> DataFrame:
  data[k] = com.apply_if_callable(v, data)
  return data
 
- def _sanitize_column(
- self, value, using_cow: bool = False
- ) -> tuple[ArrayLike, BlockValuesRefs | None]:
+ def _sanitize_column(self, value) -> tuple[ArrayLike, BlockValuesRefs | None]:
  """
  Ensures new columns (which go into the BlockManager as new blocks) are
  always copied (or a reference is being tracked to them under CoW)
@@ -4907,7 +4902,7 @@ def _sanitize_column(
  if is_dict_like(value):
  if not isinstance(value, Series):
  value = Series(value)
- return _reindex_for_setitem(value, self.index, using_cow=using_cow)
+ return _reindex_for_setitem(value, self.index)
 
  if is_list_like(value):
  com.require_length_match(value, self.index)
@@ -11923,12 +11918,12 @@ def _from_nested_dict(data) -> collections.defaultdict:
 
 
 def _reindex_for_setitem(
- value: DataFrame | Series, index: Index, using_cow: bool = False
+ value: DataFrame | Series, index: Index
 ) -> tuple[ArrayLike, BlockValuesRefs | None]:
  # reindex if necessary
 
  if value.index.equals(index) or not len(index):
- if using_cow and isinstance(value, Series):
+ if using_copy_on_write() and isinstance(value, Series):
  return value._values, value._references
  return value._values.copy(), None