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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 register_watcher(self, event_type, callback, register_timeout=None):
 """ Register a callback for a given event type. """

        self._push_watchers[event_type].add(callback)
        self.wait_for_response(
            RegisterMessage(event_list=[event_type]),
            timeout=register_timeout)