conv.py

import argparse

class Value(object):
    def __init__(self, alias, name):
        super(Value, self).__init__()
        self.alias = alias
        self.name = name
    
class Converter(object):
    def __init__(self, first, second, multiplier):
        super(Converter, self).__init__()
        self.first = first
        self.second = second
        self.multiplier = multiplier

    def alias(self):
        return "%s-%s" % (self.first.alias, self.second.alias)
        
    def help(self):
        return "convert 1 %s to %f %s" % (self.first.name, self.multiplier, self.second.name)

    def info(self):
        return "%s %s to %s" % (self.alias(), self.first.name, self.second.name)

    def convert(self, value):
        return value * self.multiplier

ML = Value("ml", "millimeter") 
SM = Value("cm", "centimeter") 
M = Value("m", "meter") 
KM = Value("km", "kilometer") 

class Converters:
    def __init__(self):
        self.converters = {}
        self.add_converter(ML, SM, 10)
        self.add_converter(ML, M, 1000)
        self.add_converter(ML, KM, 100000)

        self.add_converter(SM, KM, 10000)
        self.add_converter(SM, M, 100)

        self.add_converter(M, KM, 1000)

    def add_converter(self, val1, val2, mult):
        alias1 = "%s-%s" % (val1, val2)
        alias2 = "%s-%s" % (val2, val1)
        self.converters[alias1] = Converter(val1, val2, 1.0/float(mult))
        self.converters[alias2] = Converter(val2, val1, mult)

    def __iter__(self):
        def sort(c):
            return c.alias()

        return sorted(self.converters.values(), key=sort).__iter__()
        
    def info(self, joiner="\n    "):
        result = []
        for c in self:
            result.append(c.info())
        return joiner.join(result)

def action(converter): 
    class Action(argparse.Action):
        def __init__(self, option_strings, dest, nargs, **kwargs):
            super(Action, self).__init__(option_strings, dest, nargs, **kwargs)

        def __call__(self, parser, namespace, values, option_string=None):
            # result = [converter.convert(val) for val in values]
            if not isinstance(values, list):
                values = [values]

            result = [converter.convert(val) for val in values]
            setattr(namespace, "result", result)
    return Action


def main():
    converters = Converters()
    parser = argparse.ArgumentParser()
    parser.add_argument("-l", "--list", help="show converter list", action="store_true")
    for c in converters:
        parser.add_argument("-" + c.alias(), nargs="*", help=c.help(),  action=action(c), type=float)

    args = parser.parse_args()
    if args.list:
        print "Available converters:\n    %s" % converters.info()

    else:
        try:
            if args.result:
                print " ".join(map(str, args.result))
        except AttributeError:
            parser.print_help()

if __name__ == "__main__":
    main()