As to the nature of the error when 'add_subparsers' is given an 'action' parameter:

'add_subparsers' does several things to 'kwargs' before it passes them to the relevant Action class.

     def add_subparsers(self, **kwargs):
        # adds 'parser_class'
        # removes 'title', 'description' (used in an argument group)
        # add 'prog'
        parsers_class = self._pop_action_class(kwargs, 'parsers')
        action = parsers_class(option_strings=[], **kwargs)

What I wrote earlier about using the registry is partly wrong.  The Action class is determined by either the 'action' parameter or the registry entry.

     In [17]: p._pop_action_class({}, 'parsers')
     Out[17]: argparse._SubParsersAction

     In [18]: p._pop_action_class({'action':'test'}, 'parsers')
     Out[18]: 'test'

So the 'action' parameter works - if you specify a compatible Action class.

    sp=p.add_subparsers(dest='cmd',action=argparse._SubParsersAction)

Such a class must have the same __init__ signature, otherwise you'll get errors such the OP's.

It might be worth rewriting the documentation line so this is clearer.  Otherwise I recommend closing this issue.






     action = parsers_class(option_strings=[], **kwargs)