code: def _get_connection(self, cluster):
          
          # w=1 because:
          # http://stackoverflow.com/questions/14798552/is-mongodb-2-x-write-concern-w-1-truly-equals-to-safe-true
          if 'connection' not in cluster:
              cluster['connection'] = self._connection_class(
                  socketTimeoutMS=self._network_timeout,
                  w=1,
                  j=self.j,
                  **cluster['params'])
  
          return cluster['connection']
docs: Return a connection to a Cluster.
  
          Return a MongoClient or a MongoReplicaSetClient for the given Cluster.
          This is done in a lazy manner (if there is already a Client connected to
          the Cluster, it is returned and no other Client is created).
  
          Args:
              cluster: A dict containing information about a cluster.
  
          Returns:
              A MongoClient or MongoReplicaSetClient instance connected to the
              desired cluster