code: def _remove_io_handler(self, handler):
          
          if handler in self._unprepared_handlers:
              old_fileno = self._unprepared_handlers[handler]
              del self._unprepared_handlers[handler]
          else:
              old_fileno = handler.fileno()
          if old_fileno is not None:
              try:
                  del self._handlers[old_fileno]
                  self.poll.unregister(old_fileno)
              except KeyError:
                  pass
docs: Remove an i/o-handler.