I've posted a file that runs your code as you expect.

It uses a custom Action class (like your test case).  It subclasses ._SubParsersAction, and replaces the 9351 namespace use with the original one.  I use the registry to change the class that parser.add_subparsers() uses.

The stock argparse.py file does not need to be changed.

ps

In your custom Action I access `namespace.foo` with `getattr(namespace, 'foo', None)` which is how argparse accesses the namespace, and does not throw attribute errors.