argparse subnamespace: Add NEWS and whatsnew sections

Author: jb2170

Doc/whatsnew/3.15.rst

@@ -835,6 +835,12 @@ argparse
   double backticks (RST inline-literal style).
   (Contributed by Hugo van Kemenade in :gh:`149375`.)
 
+* Added ``subnamespace`` keyword-only flag to
+  :meth:`argparse._SubParsersAction.add_parser` to allow nested
+  :class:`argparse.Namespace`\ s, which correspond with the hierarchical
+  nature of subparsers. By default ``subnamespace`` is ``False`` for
+  backwards compatibility.
+
 
 array
 -----

Misc/NEWS.d/next/Library/2026-05-02-11-22-02.gh-issue-103639.l9hA7f.rst

@@ -0,0 +1,5 @@
+Add ``subnamespace`` keyword-only flag to
+:meth:`argparse._SubParsersAction.add_parser` to allow nested
+:class:`argparse.Namespace`\ s, which correspond with the hierarchical
+nature of subparsers. By default ``subnamespace`` is ``False`` for
+backwards compatibility.