import argparse

class MyParserAction(argparse._SubParsersAction):
    def __call__(self, parser, namespace, values, option_string=None):
        parser_name = values[0]
        arg_strings = values[1:]

        # set the parser name if requested
        if self.dest is not argparse.SUPPRESS:
            setattr(namespace, self.dest, parser_name)

        # select the parser
        try:
            parser = self._name_parser_map[parser_name]
        except KeyError:
            args = {'parser_name': parser_name,
                    'choices': ', '.join(self._name_parser_map)}
            msg = _('unknown parser %(parser_name)r (choices: %(choices)s)') % args
            raise ArgumentError(self, msg)

        # parse all the remaining options into the namespace
        # store any unrecognized options on the object, so that the top
        # level parser can decide what to do with them

        # ORIGINAL 
        namespace, arg_strings = parser.parse_known_args(arg_strings, namespace)

        # issue 9351 change
        ## In case this subparser defines new defaults, we parse them
        ## in a new namespace object and then update the original
        ## namespace for the relevant parts.
        #subnamespace, arg_strings = parser.parse_known_args(arg_strings, None)
        #for key, value in vars(subnamespace).items():
        #    setattr(namespace, key, value)

        if arg_strings:
            vars(namespace).setdefault(_UNRECOGNIZED_ARGS_ATTR, [])
            getattr(namespace, _UNRECOGNIZED_ARGS_ATTR).extend(arg_strings)

# issue 27859 example

p = argparse.ArgumentParser()

# chg which Action class is used for subparsers
p.register('action', 'parsers', MyParserAction)

p.add_argument('--foo', action='store_true', default=False)
sp = p.add_subparsers()
sp1 = sp.add_parser('first')

# Add custom action
class BarAction(argparse.Action):
    def __call__(self, parser, namespace, value, option_string=None):
        """ Try and access the `foo` variable """
        # Just to highlight the current namespace
        print(namespace)
        # Perform action dependent on foo (this will error out as
        # the parent name-space is not passed... :()
        # if namespace.foo:
        # better access namespace as rest of argparse does
        foo = getattr(namespace, 'foo', None)
        print('foo:',foo)
        
sp1.add_argument('--bar', nargs=0, action=BarAction)
args = p.parse_args()
print(args)

"""
# with new class:
0906:~/mypy/argdev$ python3 issue27859test.py first --bar
Namespace(bar=None, foo=False)
foo: False
Namespace(bar=None, foo=False)
0909:~/mypy/argdev$ python3 issue27859test.py --foo first --bar
Namespace(bar=None, foo=True)
foo: True
Namespace(bar=None, foo=True)

# without registry change
0909:~/mypy/argdev$ python3 issue27859test.py first --bar
Namespace(bar=None)
foo: None
Namespace(bar=None, foo=False)
0910:~/mypy/argdev$ python3 issue27859test.py --foo first --bar
Namespace(bar=None)
foo: None
Namespace(bar=None, foo=True)


"""