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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 heartbeat(self):
 """Heartbeat request to keep session alive. """

        unique_id = self.new_unique_id()
        message = {
            'op': 'heartbeat',
            'id': unique_id,
        }
        self._send(message)
        return unique_id