How to add custom parameter into Python logging formatter?

u can do it with logging.LoggerAdapter

myLogger = logging.LoggerAdapter(logging.getLogger("my-logger"), {"username" : get_user_name()})

Here is the complete solution for your program. I use a dict to build my configuration. It is better, if you have more logger

    def get_user_name():
        return session.get("username", "")

    LOGGING = {
        'version': 1,
        'disable_existing_loggers': True,
        'formatters': {
            'my_format': {
                'format': '%(username)s - %(asctime)s %(levelname)-10s %(message)s  [in %(pathname)s:%(lineno)d]'
            },
        },
        'handlers': {
            'my_handler': {
                'level': 'DEBUG',
                'class': 'logging.handlersRotatingFileHandler',
                'filename': fname,
                'maxBytes': 1 * 1024 * 1024,
                'backupCount': 5,
            },
        },
        'loggers': {
            'my_logger': {
                'handlers': ['my_handler'],
                'propagate': True,
                'level': 'DEBUG',
            },
        } }

logging.config.dictConfig(LOGGING) 
logging.LoggerAdapter(logging.getLogger('my_logger'),
                                           {"username" : get_user_name()})

Isn't the easier way to just subclass a Formatter and add your custom attribute to the LogRecord just before formatting?

For instance I use this code:

# A custom formatter to add the timestamp from the simulated clock.
class _Formatter(logging.Formatter):
  def format(self, record):
    record.simulated_clock = clock.get_time()
    return super(_Formatter, self).format(record)

# Creates a logger object.
def _create_logger():
  logger = logging.getLogger("simulation")
  logger.setLevel(kLevel)
  ch = logging.StreamHandler()
  ch.setLevel(kLevel)
  formatter = _Formatter("%(simulated_clock)s - %(levelname)s - %(message)s")
  ch.setFormatter(formatter)
  logger.addHandler(ch)

  return logger

I don't consider myself a Python expert, but it works for me....