Return a new DStream in which each RDD has a single element
        generated by reducing each RDD of this DStream.

def reduce(self, func):
        """
        Return a new DStream in which each RDD has a single element
        generated by reducing each RDD of this DStream.
        """
        return self.map(lambda x: (None, x)).reduceByKey(func, 1).map(lambda x: x[1])