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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 get_primary(self):
 """Return primary's address or None."""

        # Implemented here in Topology instead of MongoClient, so it can lock.
        with self._lock:
            topology_type = self._description.topology_type
            if topology_type != TOPOLOGY_TYPE.ReplicaSetWithPrimary:
                return None

            return writable_server_selector(self._new_selection())[0].address