I submitted a patch to http://bugs.python.org/issue9253

The reason why subparser became optional in the latest release is that a old test for missing positionals was phased out, and new one added.  Subparsers have always had a default 'required=False', but the old test did not actually pay attention to that attribute.

It is easy to change the default attribute, but that ends up exposing a bug in the new test (trying to ','.join[...,None,...]).

My patch corrects that, and exposes the 'required' attribute.  It also has doc changes and test additions.

I could rewrite it so it does not change the API or the doc.