restructure to focus on string dtype changes/fixes

Author: jorisvandenbossche

doc/source/whatsnew/v2.3.1.rst

@@ -9,22 +9,12 @@ including other versions of pandas.
 {{ header }}
 
 .. ---------------------------------------------------------------------------
-.. _whatsnew_231.enhancements:
-
-Enhancements
-~~~~~~~~~~~~
--
-
-.. ---------------------------------------------------------------------------
-.. _whatsnew_231.notable_bug_fixes:
-
-Notable bug fixes
-~~~~~~~~~~~~~~~~~
-
-These are bug fixes that might have notable behavior changes.
+.. _whatsnew_231.string_fixes:
 
+Improvements and fixes for the StringDtype
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-.. _whatsnew_231.notable_bug_fixes.string_comparisons:
+.. _whatsnew_231.string_fixes.string_comparisons:
 
 Comparisons between different string dtypes
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -39,16 +29,36 @@ in determining the result dtype when there are different string dtypes compared.
 - When ``pd.StringDtype("python", na_value=pd.NA)`` is compared against ``pd.StringDtype("pyarrow", na_value=np.nan)``, the result will be ``boolean``, the NumPy-backed nullable extension array.
 - When ``pd.StringDtype("python", na_value=pd.NA)`` is compared against ``pd.StringDtype("python", na_value=np.nan)``, the result will be ``boolean``, the NumPy-backed nullable extension array.
 
+.. _whatsnew_231.string_fixes.ignore_empty:
+
+Index set operations ignore empty RangeIndex and object dtype Index
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When enabling the ``future.infer_string`` option, :class:`Index` set operations (like
+union or intersection) will now ignore the dtype of an empty :class:`RangeIndex` or
+empty :class:`Index` with ``object`` dtype when determining the dtype of the resulting
+Index (:issue:`60797`).
 
-.. _whatsnew_231.api_changes:
+This ensures that combining such empty Index with strings will infer the string dtype
+correctly, rather than defaulting to ``object`` dtype. For example:
 
-API changes
-~~~~~~~~~~~
+.. code-block:: python
 
-- When enabling the ``future.infer_string`` option, :class:`Index` set operations (like
-  union or intersection) will now ignore the dtype of an empty :class:`RangeIndex` or
-  empty :class:`Index` with ``object`` dtype when determining the dtype of the resulting
-  Index (:issue:`60797`)
+    >>> pd.options.mode.infer_string = True
+    >>> df = pd.DataFrame()
+    >>> df.columns.dtype
+    dtype('int64')               # default RangeIndex for empty columns
+    >>> df["a"] = [1, 2, 3]
+    >>> df.columns.dtype
+    <StringDtype(na_value=nan)>  # new columns use string dtype instead of object dtype
+
+.. _whatsnew_231.string_fixes.bugs:
+
+Bug fixes
+^^^^^^^^^
+- Bug in :meth:`.DataFrameGroupBy.min`, :meth:`.DataFrameGroupBy.max`, :meth:`.Resampler.min`, :meth:`.Resampler.max` where all NA values of string dtype would return float instead of string dtype (:issue:`60810`)
+- Bug in :meth:`DataFrame.sum` with ``axis=1``, :meth:`.DataFrameGroupBy.sum` or :meth:`.SeriesGroupBy.sum` with ``skipna=True``, and :meth:`.Resampler.sum` with all NA values of :class:`StringDtype` resulted in ``0`` instead of the empty string ``""`` (:issue:`60229`)
+- Fixed bug in :meth:`DataFrame.explode` and :meth:`Series.explode` where methods would fail with ``dtype="str"`` (:issue:`61623`)
 
 
 .. _whatsnew_231.regressions:
@@ -62,9 +72,7 @@ Fixed regressions
 
 Bug fixes
 ~~~~~~~~~
-- Bug in :meth:`.DataFrameGroupBy.min`, :meth:`.DataFrameGroupBy.max`, :meth:`.Resampler.min`, :meth:`.Resampler.max` where all NA values of string dtype would return float instead of string dtype (:issue:`60810`)
-- Bug in :meth:`DataFrame.sum` with ``axis=1``, :meth:`.DataFrameGroupBy.sum` or :meth:`.SeriesGroupBy.sum` with ``skipna=True``, and :meth:`.Resampler.sum` with all NA values of :class:`StringDtype` resulted in ``0`` instead of the empty string ``""`` (:issue:`60229`)
-- Fixed bug in :meth:`DataFrame.explode` and :meth:`Series.explode` where methods would fail with ``dtype="str"`` (:issue:`61623`)
+-
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_231.other: