def _doWriteOrRead(self, selectable, fd, event):
        """
        Private method called when a FD is ready for reading, writing or was
        lost. Do the work and raise errors where necessary.
        """
        why = None
        inRead = False
        (filter, flags, data, fflags) = (
            event.filter, event.flags, event.data, event.fflags)

        if flags & KQ_EV_EOF and data and fflags:
            why = main.CONNECTION_LOST
        else:
            try:
                if selectable.fileno() == -1:
                    inRead = False
                    why = posixbase._NO_FILEDESC
                else:
                    if filter == KQ_FILTER_READ:
                        inRead = True
                        why = selectable.doRead()
                    if filter == KQ_FILTER_WRITE:
                        inRead = False
                        why = selectable.doWrite()
            except:
                # Any exception from application code gets logged and will
                # cause us to disconnect the selectable.
                why = failure.Failure()
                log.err(why, "An exception was raised from application code" \
                             " while processing a reactor selectable")

        if why:
            self._disconnectSelectable(selectable, why, inRead)

        Private method called when a FD is ready for reading, writing or was
        lost. Do the work and raise errors where necessary.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/kqreactor.py