def __call__(self, event):
        """
        Forward events to the legacy observer after editing them to
        ensure compatibility.

        @param event: an event
        @type event: L{dict}
        """

        # The "message" key is required by textFromEventDict()
        if "message" not in event:
            event["message"] = ()

        if "time" not in event:
            event["time"] = event["log_time"]

        if "system" not in event:
            event["system"] = event.get("log_system", "-")

        # Format new style -> old style
        if "format" not in event and event.get("log_format", None) is not None:
            # Create an object that implements __str__() in order to defer the
            # work of formatting until it's needed by a legacy log observer.
            event["format"] = "%(log_legacy)s"
            event["log_legacy"] = StringifiableFromEvent(event.copy())

            # In the old-style system, the 'message' key always holds a tuple
            # of messages. If we find the 'message' key here to not be a
            # tuple, it has been passed as new-style parameter. We drop it
            # here because we render it using the old-style 'format' key,
            # which otherwise doesn't get precedence, and the original event
            # has been copied above.
            if not isinstance(event["message"], tuple):
                event["message"] = ()

        # From log.failure() -> isError blah blah
        if "log_failure" in event:
            if "failure" not in event:
                event["failure"] = event["log_failure"]
            if "isError" not in event:
                event["isError"] = 1
            if "why" not in event:
                event["why"] = formatEvent(event)
        elif "isError" not in event:
            if event["log_level"] in (LogLevel.error, LogLevel.critical):
                event["isError"] = 1
            else:
                event["isError"] = 0

        self.legacyObserver(event)

        Forward events to the legacy observer after editing them to
        ensure compatibility.

        @param event: an event
        @type event: L{dict}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/logger/_legacy.py