How do I add custom field to Python log format string?

You could use a LoggerAdapter so you don't have to pass the extra info with every logging call:

import logging
extra = {'app_name':'Super App'}

logger = logging.getLogger(__name__)
syslog = logging.StreamHandler()
formatter = logging.Formatter('%(asctime)s %(app_name)s : %(message)s')
syslog.setFormatter(formatter)
logger.setLevel(logging.INFO)
logger.addHandler(syslog)

logger = logging.LoggerAdapter(logger, extra)
logger.info('The sky is so blue')

logs (something like)

2013-07-09 17:39:33,596 Super App : The sky is so blue

---

Filters can also be used to add contextual information.

import logging

class AppFilter(logging.Filter):
    def filter(self, record):
        record.app_name = 'Super App'
        return True

logger = logging.getLogger(__name__)
logger.addFilter(AppFilter())
syslog = logging.StreamHandler()
formatter = logging.Formatter('%(asctime)s %(app_name)s : %(message)s')
syslog.setFormatter(formatter)
logger.setLevel(logging.INFO)
logger.addHandler(syslog)

logger.info('The sky is so blue')

produces a similar log record.

Python3

As of Python3.2 you can now use LogRecordFactory

import logging

logging.basicConfig(format="%(custom_attribute)s - %(message)s")

old_factory = logging.getLogRecordFactory()

def record_factory(*args, **kwargs):
    record = old_factory(*args, **kwargs)
    record.custom_attribute = "my-attr"
    return record

logging.setLogRecordFactory(record_factory)

>>> logging.info("hello")
my-attr - hello

Of course, record_factory can be customized to be any callable and the value of custom_attribute could be updated if you keep a reference to the factory callable.

Why is that better than using Adapters / Filters?

• You do not need to pass your logger around the application
• It actually works with 3rd party libraries that use their own logger (by just calling logger = logging.getLogger(..)) would now have the same log format. (this is not the case with Filters / Adapters where you need to be using the same logger object)
• You can stack/chain multiple factories