code: def combineByKey(self, createCombiner, mergeValue, mergeCombiners,
                       numPartitions=None):
          
          if numPartitions is None:
              numPartitions = self._sc.defaultParallelism
  
          def func(rdd):
              return rdd.combineByKey(createCombiner, mergeValue, mergeCombiners, numPartitions)
          return self.transform(func)
docs: Return a new DStream by applying combineByKey to each RDD.