pythonloggingpython-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.

Solution

  • 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?