def _ioEventCallback(self, source, condition):
        """
        Called by event loop when an I/O event occurs.
        """
        log.callWithLogger(
            source, self._doReadOrWrite, source, source, condition)
        return True  # True = don't auto-remove the source

        Called by event loop when an I/O event occurs.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/_glibbase.py