How to input variables in logger formatter?

Question

I currently have:

FORMAT = '%(asctime)s - %(levelname)s - %(message)s'
logging.basicConfig(format=FORMAT, datefmt='%d/%m/%Y %H:%M:%S', filename=LOGFILE, level=getattr(logging, options.loglevel.upper()))

... which works great, however I'm trying to do:

FORMAT = '%(MYVAR)s %(asctime)s - %(levelname)s - %(message)s'

and that just throws keyerrors, even though MYVAR is defined.

Is there a workaround? MYVAR is a constant, so it would be a shame of having to pass it everytime I invoke the logger.

Thank you!

Answer 1

You could use a custom filter:

import logging

MYVAR = 'Jabberwocky'


class ContextFilter(logging.Filter):
    """
    This is a filter which injects contextual information into the log.
    """
    def filter(self, record):
        record.MYVAR = MYVAR
        return True

FORMAT = '%(MYVAR)s %(asctime)s - %(levelname)s - %(message)s'
logging.basicConfig(format=FORMAT, datefmt='%d/%m/%Y %H:%M:%S')

logger = logging.getLogger(__name__)
logger.addFilter(ContextFilter())

logger.warning("'Twas brillig, and the slithy toves")

yields

Jabberwocky 24/04/2013 20:57:31 - WARNING - 'Twas brillig, and the slithy toves

Answer 2

You could use a custom Filter, as unutbu says, or you could use a LoggerAdapter:

import logging

logger = logging.LoggerAdapter(logging.getLogger(__name__), {'MYVAR': 'Jabberwocky'})

FORMAT = '%(MYVAR)s %(asctime)s - %(levelname)s - %(message)s'
logging.basicConfig(format=FORMAT, datefmt='%d/%m/%Y %H:%M:%S')

logger.warning("'Twas brillig, and the slithy toves")

which gives

Jabberwocky 25/04/2013 07:39:52 - WARNING - 'Twas brillig, and the slithy toves

Alternatively, just pass the information with every call:

import logging

logger = logging.getLogger(__name__)

FORMAT = '%(MYVAR)s %(asctime)s - %(levelname)s - %(message)s'
logging.basicConfig(format=FORMAT, datefmt='%d/%m/%Y %H:%M:%S')

logger.warning("'Twas brillig, and the slithy toves", extra={'MYVAR': 'Jabberwocky'})

which gives the same result.

Since MYVAR is practically constant, the LoggerAdapter approach requires less code than the Filter approach in your case.