["python","logging","python-logging"]
How do I add custom field to Python log format string?

My current format string is: formatter = logging.Formatter('%(asctime)s : %(message)s') and I want to add a new field called app_name which will have a different value in each script that contains this formatter. import logging formatter = logging.Formatter('%(asctime)s %(app_name)s : %(message)s') syslog.setFormatter(formatter) logger.addHandler(syslog) But I'm not sure how to pass that app_name value to the logger to interpolate into the format string. I can obviously get it to appear in the log message by passing it each time but this is messy. I've tried: logging.info('Log message', app_name='myapp') logging.info('Log message', {'app_name', 'myapp'}) logging.info('Log message', 'myapp') but none work.
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