``acl_users`` can be either a set or a list. As long as Python sets are not JSON serializable, cast them to lists in functional tests. This way we avoid gate failures like [1]. [1] http://logs.openstack.org/46/319446/7/check/gate-python-barbicanclient-devstack-dsvm/cb80e0c/console.html#_2016-08-04_15_38_51_649177 Change-Id: I6b8601f7168120cf0e15716cc22b8834ddb127e8