code: def ensure_one_opt(opt, parser, opt_list):
      
  
      the_one = None
      for name in opt_list:
          attr = name[2:].replace('-', '_')
          if hasattr(opt, attr) and (getattr(opt, attr) is not None):
              if the_one is None:
                  the_one = name
              else:
                  parser.error("%s and %s are mutually exculsive" \
                                % (the_one, name))
  
      if the_one is None:
          parser.error("you must supply one of the following %s" \
                        % (', '.join(opt_list)))
docs: Check that one and only one in the opt_list is defined in opt
  
      Parameters
      ----------
      opt : object
          Result of option parsing
      parser : object
          OptionParser instance.
      opt_list : list of strings