def mainLoop(self):
        """
        Run the runner (C{CFRunLoopRun} or something that calls it), which runs
        the run loop until C{crash()} is called.
        """
        self._inCFLoop = True
        try:
            self._runner()
        finally:
            self._inCFLoop = False

        Run the runner (C{CFRunLoopRun} or something that calls it), which runs
        the run loop until C{crash()} is called.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/cfreactor.py