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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 get_queue_for( self, queue_identifier: QueueIdentifier, ) -> NotifyingQueue: """ Return the queue identified by the given queue identifier. If the queue doesn't exist it will be instantiated. """

        queue = self.queueids_to_queues.get(queue_identifier)

        if queue is None:
            items: List[QueueItem_T] = list()
            queue = self.init_queue_for(queue_identifier, items)

        return queue