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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 check_down_connections(self):
 """ Iterates through all connections which were previously listed as unavailable and marks any that have expired their retry_timeout as being up. """

        now = time.time()

        for db_num, marked_down_at in self._down_connections.items():
            if marked_down_at + self.retry_timeout <= now:
                self.mark_connection_up(db_num)