code: def countByWindow(self, windowDuration, slideDuration):
          
          return self.map(lambda x: 1).reduceByWindow(operator.add, operator.sub,
                                                      windowDuration, slideDuration)
docs: Return a new DStream in which each RDD has a single element generated
          by counting the number of elements in a window over this DStream.
          windowDuration and slideDuration are as defined in the window() operation.
  
          This is equivalent to window(windowDuration, slideDuration).count(),
          but will be more efficient if window is large.