My goal is to be able to access the extra argument of each LogRecord to format it properly, as a way to create a basic structured logger.

logger.info("my message", extra={"a": 1}  # I will format this to my needs

Since the extra argument is unpacked to record.__dict__, I thought about patching the record at creation time to save it in specific attribute. I am able to do that with the following code:

class LoggerWithExtra(logging.Logger):
    """
    Add the 'extra' argument to log record to be able to access it in the formatter.
    By default, the 'extra' argument is stored in record.__dict__ and is polluted with other attributes.
    """

    def makeRecord(self, *args, **kwargs):
        record = super(LoggerWithExtra, self).makeRecord(*args, **kwargs)
        record._extra = args[8]  # the 'extra' argument
        return record


logging.setLoggerClass(LoggerWithExtra)

I then format the records like this:

class MyFormatter(logging.Formatter):
    def format(self, record: logging.LogRecord) -> str:
        rec = {}

        timestamp = time.strftime("%Y-%m-%dT%H:%M:%S", time.localtime(record.created))
        rec["timestamp"] = timestamp + "%03dZ" % record.msecs

        rec["severity"] = record.levelname
        rec["message"] = record.getMessage()

        extra = getattr(record, "_extra", None)
        if extra is not None:
            rec.update(**extra)

        return json.dumps(rec)

However, seems like the best practice is to use logging.setLogRecordFactory(record_factory). I then tried this:

old_factory = logging.getLogRecordFactory()


def record_factory(*args, **kwargs):
    record = old_factory(*args, **kwargs)
    record._extra = args[8]
    return record


logging.setLogRecordFactory(record_factory)

But record._extra = args[8] seems to always be None. What am I missing here?