<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def update_callback(self, fd, handle, events):
 """Update the event mask associated with an existing callback. If you want to temporarily disable a callback then you can use this method with an *events* argument of ``0``. This is more efficient than removing the callback and adding it again later. """

        if self._mpoll is None:
            raise RuntimeError('Poller instance is closed')
        mpoll = self._mpoll.get(fd)
        if mpoll is None:
            raise ValueError('not watching fd {}'.format(fd))
        mpoll.update_callback(handle, events)