<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _trigger(self):
 """Trigger all callbacks registered to this Future. This method is called internally by the batch once the batch completes. Args: message_id (str):
 The message ID, as a string. """

        self._completed.set()
        for callback in self._callbacks:
            callback(self)