openstack same_host & different_host does not take effect if specify only one uuid for the hint

I was using same_host and different_host as schedule hint when create new instance, but found that same_host and different_host does not take effect if only specify only one uuid for hint.

The case is as following:
1) Create a new instance and the instance uuid is ff81ed99-78bb-4285-87a0-33e12a919ebc
2) Create another instance and I want the new instance running on the same host with the first instance.
nova boot --image 796503bf-536a-4bbf-b34c-f7b302636ba4 --flavor 1 myserver --hint same_host=ff81ed99-78bb-4285-87a0-33e12a919ebc

The result is that the new instance will failed because nova cannot find a valid host, checking code for the filter.

class SameHostFilter(AffinityFilter):
    '''Schedule the instance on the same host as another instance in a set of
    of instances.
    '''

    def host_passes(self, host_state, filter_properties):
        context = filter_properties['context']
        scheduler_hints = filter_properties['scheduler_hints']
        me = host_state.host

        affinity_uuids = scheduler_hints.get('same_host', [])
        if affinity_uuids:
            return any([i for i
                          in affinity_uuids
                          if self._affinity_host(context, i) == me]) <<<<<<<<<< Here caused the problem. If only one uuid, then the filter will take affinity_uuids as f,f,8,1,e....., this caused the scheduler failed to find a target host
        # With no same_host key
        return True