def _validate_event_listeners(option, listeners):
    if not isinstance(listeners, Sequence):
        raise TypeError("%s must be a list or tuple" % (option,))
    for listener in listeners:
        if not isinstance(listener, CommandListener):
            raise TypeError("Only subclasses of "
                            "pymongo.monitoring.CommandListener are supported")
    return listeners
step should be zero for the first form
dataset/ETHPy150Open mongodb/mongo-python-driver/pymongo/monitoring.py/_validate_event_listeners--crossover with--dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-1.4/django/contrib/formtools/tests/__init__.py/WizardTests.test_step_starts_at_zero