def __call__(self, event):
        """
        Forward to next observer if predicate allows it.
        """
        if self._shouldLogEvent(event):
            if "log_trace" in event:
                event["log_trace"].append((self, self.observer))
            self._observer(event)
        else:
            self._negativeObserver(event)

        Forward to next observer if predicate allows it.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/logger/_filter.py