code: def union(self, other):
          
          if self._slideDuration != other._slideDuration:
              raise ValueError("the two DStream should have same slide duration")
          return self.transformWith(lambda a, b: a.union(b), other, True)
docs: Return a new DStream by unifying data of another DStream with this DStream.
  
          @param other: Another DStream having the same interval (i.e., slideDuration)
                       as this DStream.