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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 on_log(self):
 # type: () -> Callable """Decorate a callback function to handle MQTT logging. **Example Usage:** :: @mqtt.on_log() def handle_logging(client, userdata, level, buf):
 print(client, userdata, level, buf) """

        def decorator(handler):
            # type: (Callable) -> Callable
            self.client.on_log = handler
            return handler

        return decorator