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

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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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