<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_partition_owner(self, partition_id):
 """ Gets the owner of the partition if it's set. Otherwise it will trigger partition assignment. :param partition_id: (int), the partition id. :return: (:class:`~hazelcast.core.Address`), owner of partition or ``None`` if it's not set yet. """

        if partition_id not in self.partitions:
            self._do_refresh()
        return self.partitions.get(partition_id, None)