code: def parse_argument(self, arg):
          
          lookup = self.casesensitive and arg or arg.lower()
          if lookup in self.special:
              return self.special[lookup]
          try:
              return self.to_python(arg, *self.args, **self.kw)
          except Exception, e:
              raise BadArgument(arg, str(e))
docs: Parse a single argument.
          
          Lookup arg in self.specials, or call .to_python() if absent. Raise 
          BadArgument on errors.