#nova/tests/network/test_manager.py
def test_create_networks_with_Neutron(self):
    CONF.set_override("network_api_class", "nova.network.neutron2.api.API")
    self.assertRaises(exception.NetworkNotCreated,
        self.network.create_networks, None)
#manager.py
if CONF.network_api_class == "nova.network.neutron2.api.API":
    raise exception.NetworkNotCreated(req="not nova.API used")