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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 round_trip_time(self):
 """The current average latency or None."""

        # This override is for unittesting only!
        if self._address in self._host_to_round_trip_time:
            return self._host_to_round_trip_time[self._address]

        return self._round_trip_time