Back ported to 2.7 (it should work with earlier versions with added argparse)

argumentor.py

import argparse
import inspect

class Argumentor:

    def __init__(self, description=None):
        self._parser = argparse.ArgumentParser(description=description)
        self._subparsers = self._parser.add_subparsers()

    def add(self, description={}):
        """
        @description - argument description dictionary
        """
        def add_decorator(func):
            subparser = self._subparsers.add_parser(func.__name__, help=func.__doc__)
            subparser.set_defaults(_func=func)
            #
            sig = inspect.getargspec(func)
            defaults = dict(zip(sig.args[-len(sig.defaults):],sig.defaults))
            for idx,name in enumerate(sig.args):
                arg_help = description.get(name) if description else "Argument #{0}".format(idx+1)
                if not name in defaults:
                    # a positional argument
                    subparser.add_argument(name, help=arg_help)
                else:
                    # an optional argument
                    arg_default = defaults.get(name)
                    subparser.add_argument("-" + name[0], "--" + name,
                            help=arg_help,
                            action="store" if arg_default is None else "store_const",
                            const=arg_default)
            return func

        return add_decorator

    def parse(self):
        args = vars(self._parser.parse_args())
        func = args.pop("func", self._parser.print_usage)
        func(**args)


if __name__ == "__main__":
    """
    Usage example:
    """
    parser = Argumentor("An Argumentor Example")

    @parser.add(dict(some_command="some kind of command", option="an option"))
    def show(some_command, option = None):
        "show something"
        print("some_command:", some_command)
        print("option:", option)

    @parser.add(dict(whatever="something completely different", const_option="eat spam"))
    def do(whatever, const_option = "spam"):
        "do something"
        print("whatever:", whatever)
        print("const_option:", const_option)

    parser.parse()