def processingFailed(self, reason):
        """
        Errback and L{Deferreds} waiting for finish notification.
        """
        if self._finishedDeferreds is not None:
            observers = self._finishedDeferreds
            self._finishedDeferreds = None
            for obs in observers:
                obs.errback(reason)

        Errback and L{Deferreds} waiting for finish notification.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/test/requesthelper.py