The function that will be executed by the scheduler.

def callback(self):
        """The function that will be executed by the scheduler."""
        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)