def _diff_and_merge_host_list(defined, existing):
    '''
    If Zabbix template is to be updated then list of assigned hosts must be provided in all or nothing manner to prevent
    some externally assigned hosts to be detached.

    :param defined: list of hosts defined in sls
    :param existing: list of hosts taken from live Zabbix
    :return: list to be updated (combinated or empty list)
    '''
    try:
        defined_host_ids = set([host['hostid'] for host in defined])
        existing_host_ids = set([host['hostid'] for host in existing])
    except KeyError:
        raise SaltException('List of hosts in template not defined correctly.')

    diff = defined_host_ids - existing_host_ids
    return [{'hostid': six.text_type(hostid)} for hostid in diff | existing_host_ids] if diff else []
If Zabbix template is to be updated then list of assigned hosts must be provided in all or nothing manner to prevent
    some externally assigned hosts to be detached.

    :param defined: list of hosts defined in sls
    :param existing: list of hosts taken from live Zabbix
    :return: list to be updated (combinated or empty list)