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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _attach(self, fcp, assigner_id, target_wwpn, target_lun, multipath, os_version, mount_point):
 """Attach a volume First, we need translate fcp into local wwpn, then dedicate fcp to the user if it's needed, after that call smt layer to call linux command """

        LOG.info('Start to attach device to %s' % assigner_id)
        self.fcp_mgr.init_fcp(assigner_id)
        new = self.fcp_mgr.increase_fcp_usage(fcp, assigner_id)
        try:
            if new:
                self._dedicate_fcp(fcp, assigner_id)

            self._add_disk(fcp, assigner_id, target_wwpn, target_lun,
                           multipath, os_version, mount_point)
        except exception.SDKBaseException as err:
            errmsg = 'rollback attach because error:' + err.format_message()
            LOG.error(errmsg)
            connections = self.fcp_mgr.decrease_fcp_usage(fcp, assigner_id)
            # if connections less than 1, undedicate the device
            if not connections:
                with zvmutils.ignore_errors():
                    self._undedicate_fcp(fcp, assigner_id)
            raise exception.SDKBaseException(msg=errmsg)
        # TODO: other exceptions?

        LOG.info('Attaching device to %s is done.' % assigner_id)