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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def start(self):
 """Start processing streams."""


        def cb():
            time_ = time.time()
            log.debug('Step {}'.format(time_))

            # run a step on all streams
            for d in self._dstreams:
                d._step(time_)

        self._pcb = PeriodicCallback(cb, self.batch_duration * 1000.0)
        self._pcb.start()
        self._on_stop_cb.append(self._pcb.stop)
        StreamingContext._activeContext = self