@property
    def dns_driver(self):
        if self._dns_driver:
            return self._dns_driver
        if not cfg.CONF.external_dns_driver:
            return
        try:
            self._dns_driver = driver.ExternalDNSService.get_instance()
            LOG.debug("External DNS driver loaded: %s",
                      cfg.CONF.external_dns_driver)
            return self._dns_driver
        except __HOLE__:
            LOG.exception(_LE("ImportError exception occurred while loading "
                              "the external DNS service driver"))
            raise dns.ExternalDNSDriverNotFound(
                driver=cfg.CONF.external_dns_driver)
dataset/ETHPy150Open openstack/neutron/neutron/db/dns_db.py/DNSDbMixin.dns_driver