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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 update(self, sensor, reading):
 """Update callback used by sensors to notify obervers of changes. Parameters sensor : :class:`katcp.Sensor` object The sensor whose value has changed. reading : (timestamp, status, value) tuple Sensor reading as would be returned by sensor.read() """

        parents = list(self._child_to_parents[sensor])
        for parent in parents:
            self.recalculate(parent, (sensor,))