code: def update_callback(self, handle, events):
          
          if self._poll is None:
              raise RuntimeError('poll instance is closed')
          if not has_callback(self, handle):
              raise ValueError('no such callback')
          if events & ~(READABLE|WRITABLE):
              raise ValueError('illegal event mask: {}'.format(events))
          if handle.extra == events:
              return
          if handle.extra & READABLE:
              self._readers -= 1
          if handle.extra & WRITABLE:
              self._writers -= 1
          if events & READABLE:
              self._readers += 1
          if events & WRITABLE:
              self._writers += 1
          handle.extra = events
          self._sync()
docs: Update the event mask for a callback.