code: def sleep(self, response=None):
          
  
          if response:
              slept = self.sleep_for_retry(response)
              if slept:
                  return
  
          self._sleep_backoff()
docs: Sleep between retry attempts.
  
          This method will respect a server's ``Retry-After`` response header
          and sleep the duration of the time requested. If that is not present, it
          will use an exponential backoff. By default, the backoff factor is 0 and
          this method will return immediately.