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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 create_subscriber(self):
        '''Create a subscriber instance using specified addresses and
        message types.
        '''
        if self.subscriber is None:
            if self.topics:
                self.subscriber = NSSubscriber(self.services, self.topics,
                                               addr_listener=True,
                                               addresses=self.addresses,
                                               nameserver=self.nameserver)
                self.recv = self.subscriber.start().recv