def _get_dns_driver():
    global DNS_DRIVER
    if DNS_DRIVER:
        return DNS_DRIVER
    if not cfg.CONF.external_dns_driver:
        return
    try:
        DNS_DRIVER = driver.ExternalDNSService.get_instance()
        LOG.debug("External DNS driver loaded: %s",
                  cfg.CONF.external_dns_driver)
        return 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/plugins/ml2/extensions/dns_integration.py/_get_dns_driver