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

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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.

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