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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 _subscribe(self):
 """ Start the subscription to the channel list. If self._keep_alive_function isn't None start timer thread to run self._keep_alive_function every self._keep_alive amount of seconds. """

        _LOGGER.info("PubNub subscribing")
        self._pubnub.subscribe().channels(CHANNELS).execute()
        if self._keep_alive_function is not None:
            threading.Timer(self._keep_alive, self._run_keep_alive).start()
        self._subscribed = True