How to add custom parameter into Python logging formatter?

Question

I'm using standard Python logging module with Flask framework. I want to write logs to file with the all records of users actions with custom parameter - %(username)s to logging.Formatter:

admin - 2013-10-11 15:11:47,033 action0
user1 - 2013-10-11 15:11:48,033 action1
user2 - 2013-10-11 15:11:49,033 action2
admin - 2013-10-11 15:11:50,033 action3

I'm using RotatingFileHandler:

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

file_handler = RotatingFileHandler(fname, maxBytes=1 * 1024 * 1024, backupCount=5)
file_handler.setLevel(logging.DEBUG)
file_handler.setFormatter(logging.Formatter(
    '%(username)s - %(asctime)s %(levelname)-10s %(message)s  [in %(pathname)s:%(lineno)d]'
)) # how to insert get_user_name() instead %(username)s?

app.logger.addHandler(file_handler)

What is the right way to do such logger? Thanks.

@xiaowl The problem is that I don't know how to insert admin into this log message. %(username)s can be in any place, if it's important. — orion_tvv
– orion_tvv, Commented Oct 11, 2013 at 12:52

julivico · Accepted Answer · 2015-05-11 08:58:31Z

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()})

josv · Accepted Answer · 2014-09-20 12:03:32Z

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....

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