human
Complete this python program:
def driver(reactor, cluster, scenario_factory, operation_factory, metric_factory, num_samples, result, output):
assistant
#Here is the Python code you requested.

    """:param reactor: Reactor to use.
    :param BenchmarkCluster cluster: Benchmark cluster.
    :param callable scenario_factory: A load scenario factory.
    :param callable operation_factory: An operation factory.
    :param callable metric_factory: A metric factory.
    :param int num_samples: Number of samples to take.
    :param result: A dictionary which will be updated with values to
    create a JSON result.
    :param output: A callable to receive the JSON structure, for
    printing or storage.
    """
    control_service = cluster.get_control_service(reactor)
    d = gather_deferreds([control_service.version(), control_service.list_nodes(), control_service.list_containers_configuration(), control_service.list_datasets_configuration()])
    def add_control_service(characteristics, result):
        version = characteristics[0]
        node_count = len(characteristics[1])
        container_count = len(characteristics[2])
        dataset_count = len(characteristics[3].datasets)
        result['control_service'] = dict(host=cluster.control_node_address().compressed, flocker_version=version[u'flocker'], node_count=node_count, container_count=container_count, dataset_count=dataset_count)
    d.addCallback(add_control_service, result)
    def run_benchmark(ignored):
        return benchmark(scenario_factory(reactor, cluster), operation_factory(reactor, cluster), metric_factory(reactor, cluster), num_samples)
    d.addCallback(run_benchmark)
    def add_samples(outputs, result):
        (samples, scenario_metrics) = outputs
        result['samples'] = samples
        if scenario_metrics:
            result['scenario']['metrics'] = scenario_metrics
        return result
    d.addCallback(add_samples, result)
    d.addCallback(output)
    return d