code: def _format_option_strings(self, option, mvarfmt=' <%s>', optsep=', '):
          
          opts = []
  
          if option._short_opts:
              opts.append(option._short_opts[0])
          if option._long_opts:
              opts.append(option._long_opts[0])
          if len(opts) > 1:
              opts.insert(1, optsep)
  
          if option.takes_value():
              metavar = option.metavar or option.dest.lower()
              opts.append(mvarfmt % metavar.lower())
  
          return ''.join(opts)
docs: Return a comma-separated list of option strings and metavars.
  
          :param option:  tuple of (short opt, long opt), e.g: ('-f', '--format')
          :param mvarfmt: metavar format string - evaluated as mvarfmt % metavar
          :param optsep:  separator