'Set a backup targetpool.
:param  targetpool: The existing primary targetpool
:type   targetpool: :class:`GCETargetPool`
:param  backup_targetpool: The existing targetpool to use for
failover traffic.
:type   backup_targetpool: :class:`GCETargetPool`
:param  failover_ratio: The percentage of healthy VMs must fall at or
below this value before traffic will be sent
to the backup targetpool (default 0.10)
:type   failover_ratio: ``float``
:return:  True if successful
:rtype:   ``bool``'

def ex_targetpool_set_backup_targetpool(self, targetpool, backup_targetpool, failover_ratio=0.1):

    region = targetpool.region.name
    name = targetpool.name
    req_data = {'target': backup_targetpool.extra['selfLink']}
    params = {'failoverRatio': failover_ratio}
    request = ('/regions/%s/targetPools/%s/setBackup' % (region, name))
    self.connection.async_request(request, method='POST', data=req_data, params=params)
    return True
