How to use the nova.exception function in nova

:param fatal: if False, will return False when an exception.Forbidden
           occurs.

        :raises nova.exception.Forbidden: if verification fails and fatal is
            True.

        :return: returns a non-False value (not necessarily "True") if
            authorized and False if not authorized and fatal is False.
        """
        if target is None:
            target = {'project_id': self.project_id,
                      'user_id': self.user_id}

        try:
            return policy.authorize(self, action, target)
        except exception.Forbidden:
            if fatal:
                raise
            return False

def _check_host_is_up(self, host):
        service = objects.Service.get_by_compute_host(self.context, host)

        if not self.servicegroup_api.service_is_up(service):
            raise exception.ComputeServiceUnavailable(host=host)

:param shared: Specifies if storage is dedicated or shared.
                       For shared storage disks split into partitions
        """
        volume_params = []
        for node in storage:

            name = node.get('drive_name', None)
            num_disks = node.get('num_drives', 1)

            if name is None:
                msg = _("drive_name not defined")
                raise exception.InvalidVolumeType(reason=msg)

            try:
                vol_type = volume_types.get_volume_type_by_name(context, name)
            except exception.NotFound:
                msg = _("invalid drive type name %s")
                raise exception.InvalidVolumeType(reason=msg % name)

            self._check_volume_type_correctness(vol_type)

            # if size field present - override disk size specified in DB
            size = int(node.get('size',
                                vol_type['extra_specs'].get('drive_size')))

            if shared:
                part_size = FLAGS.vsa_part_size_gb
                total_capacity = num_disks * size
                num_volumes = total_capacity / part_size
                size = part_size
            else:
                num_volumes = num_disks

def create(self, label=None, cidr=None, num_networks=None,
               network_size=None, multi_host=None, vlan=None,
               vlan_start=None, vpn_start=None, cidr_v6=None, gateway=None,
               gateway_v6=None, bridge=None, bridge_interface=None,
               dns1=None, dns2=None, project_id=None, priority=None,
               uuid=None, fixed_cidr=None):
        """Creates fixed IPs for host by range."""

        # NOTE(gmann): These checks are moved here as API layer does all these
        # validation through JSON schema.
        if not label:
            raise exception.NetworkNotCreated(req="label")
        if len(label) > 255:
            raise exception.LabelTooLong()
        if not (cidr or cidr_v6):
            raise exception.NetworkNotCreated(req="cidr or cidr_v6")

        kwargs = {k: v for k, v in locals().items()
                  if v and k != "self"}
        if multi_host is not None:
            kwargs['multi_host'] = multi_host == 'T'
        net_manager = importutils.import_object(CONF.network_manager)
        net_manager.create_networks(context.get_admin_context(), **kwargs)

self.address = node['pm_address']
        self.user = node['pm_user']
        self.password = node['pm_password']
        self.port = node['terminal_port']

        if self.node_id == None:
            raise exception.InvalidParameterValue(_("Node id not supplied "
                "to IPMI"))
        if self.address == None:
            raise exception.InvalidParameterValue(_("Address not supplied "
                "to IPMI"))
        if self.user == None:
            raise exception.InvalidParameterValue(_("User not supplied "
                "to IPMI"))
        if self.password == None:
            raise exception.InvalidParameterValue(_("Password not supplied "
                "to IPMI"))

def _volume_driver_method(self, method_name, connection_info,
                             *args, **kwargs):
        driver_type = connection_info.get('driver_volume_type')
        if driver_type not in self.volume_drivers:
            raise exception.VolumeDriverNotFound(driver_type=driver_type)
        driver = self.volume_drivers[driver_type]
        method = getattr(driver, method_name)
        return method(connection_info, *args, **kwargs)

policies = set(('anti-affinity', 'affinity', 'soft-affinity',
                    'soft-anti-affinity'))
    if group.policy in policies:
        if not _SUPPORTS_AFFINITY and 'affinity' == group.policy:
            msg = _("ServerGroupAffinityFilter not configured")
            LOG.error(msg)
            raise exception.UnsupportedPolicyException(reason=msg)
        if not _SUPPORTS_ANTI_AFFINITY and 'anti-affinity' == group.policy:
            msg = _("ServerGroupAntiAffinityFilter not configured")
            LOG.error(msg)
            raise exception.UnsupportedPolicyException(reason=msg)
        if (not _SUPPORTS_SOFT_AFFINITY and 'soft-affinity' == group.policy):
            msg = _("ServerGroupSoftAffinityWeigher not configured")
            LOG.error(msg)
            raise exception.UnsupportedPolicyException(reason=msg)
        if (not _SUPPORTS_SOFT_ANTI_AFFINITY and
                'soft-anti-affinity' == group.policy):
            msg = _("ServerGroupSoftAntiAffinityWeigher not configured")
            LOG.error(msg)
            raise exception.UnsupportedPolicyException(reason=msg)
        group_hosts = set(group.get_hosts())
        user_hosts = set(user_group_hosts) if user_group_hosts else set()
        return GroupDetails(hosts=user_hosts | group_hosts,
                            policy=group.policy, members=group.members)

availability_zone = vol.get('availability_zone', None)

        try:
            new_volume = self.volume_api.create(
                context,
                size,
                vol.get('display_name'),
                vol.get('display_description'),
                snapshot=snapshot,
                volume_type=vol_type,
                metadata=metadata,
                availability_zone=availability_zone
                )
        except exception.InvalidInput as err:
            raise exc.HTTPBadRequest(explanation=err.format_message())
        except exception.OverQuota as err:
            raise exc.HTTPForbidden(explanation=err.format_message())
        # TODO(vish): Instance should be None at db layer instead of
        #             trying to lazy load, but for now we turn it into
        #             a dict to avoid an error.
        retval = _translate_volume_detail_view(context, dict(new_volume))
        result = {'volume': retval}

        location = '%s/%s' % (req.url, new_volume['id'])

        return wsgi.ResponseObject(result, headers=dict(location=location))

def address_to_hosts(addresses):
        """Iterate over hosts within an address range.

        If an explicit range specifier is missing, the parameter is
        interpreted as a specific individual address.
        """
        try:
            return [netaddr.IPAddress(addresses)]
        except ValueError:
            net = netaddr.IPNetwork(addresses)
            if net.size < 4:
                reason = _("/%s should be specified as single address(es) "
                           "not in cidr format") % net.prefixlen
                raise exception.InvalidInput(reason=reason)
            elif net.size >= 1000000:
                # NOTE(dripton): If we generate a million IPs and put them in
                # the database, the system will slow to a crawl and/or run
                # out of memory and crash.  This is clearly a misconfiguration.
                reason = _("Too many IP addresses will be generated.  Please "
                           "increase /%s to reduce the number generated."
                          ) % net.prefixlen
                raise exception.InvalidInput(reason=reason)
            else:
                return net.iter_hosts()

def _get_vm_serial_port_mapping(self):
        serial_port_conns = self._vmutils.get_vm_serial_port_connections(
            self._instance_name)

        if not serial_port_conns:
            err_msg = _("No suitable serial port pipe was found "
                        "for instance %(instance_name)s")
            raise exception.NovaException(
                err_msg % {'instance_name': self._instance_name})

        serial_port_mapping = {}
        # At the moment, we tag the pipes by using a pipe path suffix
        # as we can't use the serial port ElementName attribute because of
        # a Hyper-V bug.
        for pipe_path in serial_port_conns:
            # expected pipe_path:
            # '\\.\pipe\fc1bcc91-c7d3-4116-a210-0cd151e019cd_rw'
            port_type = pipe_path[-2:]
            if port_type in [constants.SERIAL_PORT_TYPE_RO,
                             constants.SERIAL_PORT_TYPE_RW]:
                serial_port_mapping[port_type] = pipe_path
            else:
                serial_port_mapping[constants.SERIAL_PORT_TYPE_RW] = pipe_path