code: def backoff_generator(self):
          
          for x in self.config.backoff_delays:
              yield x
          while True:
              yield self.config.backoff_delays[-1]
docs: Generate a series of integers used for the length of the sleep
          between retries.  It produces after exhausting the list, it repeats
          the last value from the list forever.  This generator will never raise
          the StopIteration exception.