def _clean_exit(self):
        """Assert that the Bus is not running in atexit handler callback."""
        if self.state != states.EXITING:
            warnings.warn(
                'The main thread is exiting, but the Bus is in the %r state; '
                'shutting it down automatically now. You must either call '
                'bus.block() after start(), or call bus.exit() before the '
                'main thread exits.' % self.state,
                RuntimeWarning,
            )
            self.exit()
Assert that the Bus is not running in atexit handler callback.
cherrypy/process/wspbus.py