code: def parse_optional(self, string):
          
          option_map = self.option_map
          if string in option_map:
              return string, option_map[string], None
          if '=' in string:
              name, value = string.split('=', 1)
              if name in option_map:
                  return name, option_map[name], value
          return None
docs: Parse string into name, option, and value (if possible).
  
          If the string is a known option name, the string, the
          corresponding option, and ``None`` will be returned.
  
          If the string has the form ``--option=<value>`` or
          ``-o=<value>``, it will be split on equals into an option name
          and value. If the option name is known, the option name, the
          corresponding option, and the value will be returned.
  
          In all other cases, ``None`` will be returned to indicate that
          the string doesn't correspond to a known option.