code: def callback(self):
          
          try:
              return self._callback()
          except:
              # All exceptions raised here will trigger the shutdown!
              s = straceback()
              self.exceptions.append(s)
  
              # This is useful when debugging
              #try:
              #    print("Exception in callback, will cancel all tasks")
              #    for task in self.flow.iflat_tasks():
              #        task.cancel()
              #except Exception:
              #    pass
  
              self.shutdown(msg="Exception raised in callback!\n" + s)
docs: The function that will be executed by the scheduler.