Backport PR #48697 on branch 1.5.x (REGR: None converted to NaN when enlarging Series) (#48745)

Backport PR #48697: REGR: None converted to NaN when enlarging Series Co-authored-by: Patrick Hoefler <61934744+phofl@users.noreply.github.com>

Author: meeseeksmachine

doc/source/whatsnew/v1.5.1.rst

@@ -14,6 +14,7 @@ including other versions of pandas.
 
 Fixed regressions
 ~~~~~~~~~~~~~~~~~
+- Fixed Regression in :meth:`Series.__setitem__` casting ``None`` to ``NaN`` for object dtype (:issue:`48665`)
 - Fixed Regression in :meth:`DataFrame.loc` when setting values as a :class:`DataFrame` with all ``True`` indexer (:issue:`48701`)
 - Regression in :func:`.read_csv` causing an ``EmptyDataError`` when using an UTF-8 file handle that was already read from (:issue:`48646`)
 - Fixed regression in :meth:`DataFrame.plot` ignoring invalid ``colormap`` for ``kind="scatter"`` (:issue:`48726`)

pandas/core/indexing.py

@@ -2108,9 +2108,14 @@ def _setitem_with_indexer_missing(self, indexer, value):
  # this preserves dtype of the value and of the object
  if not is_scalar(value):
  new_dtype = None
+
  elif is_valid_na_for_dtype(value, self.obj.dtype):
- value = na_value_for_dtype(self.obj.dtype, compat=False)
+ if not is_object_dtype(self.obj.dtype):
+ # Every NA value is suitable for object, no conversion needed
+ value = na_value_for_dtype(self.obj.dtype, compat=False)
+
  new_dtype = maybe_promote(self.obj.dtype, value)[0]
+
  elif isna(value):
  new_dtype = None
  elif not self.obj.empty and not is_object_dtype(self.obj.dtype):

pandas/tests/series/indexing/test_setitem.py

@@ -562,6 +562,14 @@ def test_setitem_enlarge_with_na(self, na, target_na, dtype, target_dtype, index
  expected = Series(expected_values, dtype=target_dtype)
  tm.assert_series_equal(ser, expected)
 
+ def test_setitem_enlargement_object_none(self, nulls_fixture):
+ # GH#48665
+ ser = Series(["a", "b"])
+ ser[3] = nulls_fixture
+ expected = Series(["a", "b", nulls_fixture], index=[0, 1, 3])
+ tm.assert_series_equal(ser, expected)
+ assert ser[3] is nulls_fixture
+
 
 def test_setitem_scalar_into_readonly_backing_data():
  # GH#14359: test that you cannot mutate a read only buffer