[Python] 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.

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