code: def mark_bool_flags_as_mutual_exclusive(flag_names, required=False,
                                          flag_values=_flagvalues.FLAGS):
    
    for flag_name in flag_names:
      if not flag_values[flag_name].boolean:
        raise _exceptions.ValidationError(
            'Flag --{} is not Boolean, which is required for flags used in '
            'mark_bool_flags_as_mutual_exclusive.'.format(flag_name))
  
    def validate_boolean_mutual_exclusion(flags_dict):
      flag_count = sum(bool(val) for val in flags_dict.values())
      if flag_count == 1 or (not required and flag_count == 0):
        return True
      raise _exceptions.ValidationError(
          '{} one of ({}) must be True.'.format(
              'Exactly' if required else 'At most', ', '.join(flag_names)))
  
    register_multi_flags_validator(
        flag_names, validate_boolean_mutual_exclusion, flag_values=flag_values)
docs: Ensures that only one flag among flag_names is True.
  
    Args:
      flag_names: [str], names of the flags.
      required: bool. If true, exactly one flag must be True. Otherwise, at most
          one flag can be True, and it is valid for all flags to be False.
      flag_values: flags.FlagValues, optional FlagValues instance where the flags
          are defined.