def parse(self, argv):
Pop, parse and return the first self.nargs items from args.

        Subclasses that desire other behavior can override this (must be 
        overridden if self.nargs is None). 

        if self.nargs > 1 a list of parsed values will be returned.

        Raise BadNumberOfArguments or BadArgument on errors.

        NOTE: args may be modified in place by this method.