Different logging levels for filehandler and display in Python

Question

I am using the logging module in Python to write debug and error messages.

I want to write to file all messages of logging.DEBUG or greater.

However, I only want to print to the screen messages of logging.WARNING or greater.

Is this possible using just one Logger and one FileHandler?

Answer 1

As it has been mentioned, handlers are so easy to create and add that you're probably better off just using two handlers. If, however, for some reason you want to stick to one, the Python logging cookbook has a section describing more or less what you want to do: logging to both console and file, but at different levels (it even shows you how to do different formatting). It does it with a single StreamHandler rather than a FileHandler, though:

import logging

# set up logging to file - see previous section for more details
logging.basicConfig(level=logging.DEBUG,
                    format='%(asctime)s %(name)-12s %(levelname)-8s %(message)s',
                    datefmt='%m-%d %H:%M',
                    filename='/temp/myapp.log',
                    filemode='w')
# define a Handler which writes INFO messages or higher to the sys.stderr
console = logging.StreamHandler()
console.setLevel(logging.INFO)
# set a format which is simpler for console use
formatter = logging.Formatter('%(name)-12s: %(levelname)-8s %(message)s')
# tell the handler to use this format
console.setFormatter(formatter)
# add the handler to the root logger
logging.getLogger('').addHandler(console)

# Now, we can log to the root logger, or any other logger. First the root...
logging.info('Jackdaws love my big sphinx of quartz.')

# Now, define a couple of other loggers which might represent areas in your
# application:

logger1 = logging.getLogger('myapp.area1')
logger2 = logging.getLogger('myapp.area2')

logger1.debug('Quick zephyrs blow, vexing daft Jim.')
logger1.info('How quickly daft jumping zebras vex.')
logger2.warning('Jail zesty vixen who grabbed pay from quack.')
logger2.error('The five boxing wizards jump quickly.')

Edit: As discussed in the comments this code still generates two handlers, but "hides" one construction through the use of basicConfig(). I would strongly encourage you to create both explicitly.

Answer 2

You can append lots of handlers to the same logger with different loglevel, but root loglevel must be as verbose as the most verbose handler. This example log messages to file with log.debug() and log.info() to console:

log = logging.getLogger("syslog2elastic")
log.setLevel(logging.DEBUG) # this must be DEBUG to allow debug messages through

console = logging.StreamHandler()
console.setLevel(logging.INFO)
formatter = logging.Formatter("%(asctime)s.%(msecs)03d - %(name)s:%(lineno)d - %(levelname)s - %(message)s", "%Y%m%d%H%M%S")
console.setFormatter(formatter)
log.addHandler(console)

fh = RotatingFileHandler(args.logfile, maxBytes=104857600, backupCount=5)
fh.setLevel(logging.DEBUG)
formatter = logging.Formatter("%(asctime)s.%(msecs)03d - %(message)s", "%Y%m%d%H%M%S")
fh.setFormatter(formatter)
log.addHandler(fh)