import argparse

class ArgumentParser(argparse.ArgumentParser):
    def regroup_by_required(self, help_groups=['required','optional']):
        # redefine the first 2 argument groups, so they contain
        # 'required' and 'not-required' actions
        # default is 'positional' and 'optional' (flagged)
        # use this after arguments have been defined but before format_help
        # subparsers will require their own regrouping
        if len(self._action_groups)>2:
            # no change if there's a user defined group(s)
            return
        groups = self._action_groups
        for i, group in enumerate(groups):
            # rename the group, and clear out previous actions
            group.title = help_groups[i]
            group._group_actions = []

        for action in self._actions:
            if action.required:
                groups[0]._group_actions.append(action)
            else:
                groups[1]._group_actions.append(action)

parser = ArgumentParser()
parser.add_argument('pos', help='required positional')
parser.add_argument('-f', '--foo', help='optional')
parser.add_argument('-g', '--goo', required=True, help='required optional')
parser.add_argument('baz', nargs='?', help='optional positional')
parser.regroup_by_required()
parser.print_help()
print(parser.parse_args('-f FOO -g GOO pos baz'.split()))

"""
usage: alt_grouping.py [-h] [-f FOO] -g GOO pos [baz]

required:
  pos                required positional
  -g GOO, --goo GOO  required optional

optional:
  -h, --help         show this help message and exit
  -f FOO, --foo FOO  optional
  baz                optional positional
Namespace(baz='baz', foo='FOO', goo='GOO', pos='pos')
"""