python logging specific level only

Question

I'm logging events in my python code uing the python logging module. I have 2 logging files I wish to log too, one to contain user information and the other a more detailed log file for devs. I've set the the two logging files to the levels I want (usr.log = INFO and dev.log = ERROR) but cant work out how to restrict the logging to the usr.log file so only the INFO level logs are written to the log file as opposed to INFO plus everthing else above it e.g. INFO, WARNING, ERROR and CRITICAL.

This is basically my code:-

import logging

logger1 = logging.getLogger('')
logger1.addHandler(logging.FileHandler('/home/tmp/usr.log')
logger1.setLevel(logging.INFO)
logger2 = logging.getLogger('')
logger2.addHandler(logging.FileHandler('/home/tmp/dev.log')
logger2.setLevel(logging.ERROR)

logging.critical('this to be logged in dev.log only')
logging.info('this to be logged to usr.log and dev.log')
logging.warning('this to be logged to dev.log only')

Any help would be great thank you.

Answer 1

I am in general agreement with David, but I think more needs to be said. To paraphrase The Princess Bride - I do not think this code means what you think it means. Your code has:

logger1 = logging.getLogger('')
...
logger2 = logging.getLogger('')

which means that logger1 and logger2 are the same logger, so when you set the level of logger2 to ERROR you actually end up setting the level of logger1 at the same time. In order to get two different loggers, you would need to supply two different logger names. For example:

logger1 = logging.getLogger('user')
...
logger2 = logging.getLogger('dev')

Worse still, you are calling the logging module's critical(), info() and warning() methods and expecting that both loggers will get the messages. This only works because you used the empty string as the name for both logger1 and logger2 and thus they are not only the same logger, they are also the root logger. If you use different names for the two loggers as I have suggested, then you'll need to call the critical(), info() and warning() methods on each logger individually (i.e. you'll need two calls rather than just one).

What I think you really want is to have two different handlers on a single logger. For example:

import logging

mylogger = logging.getLogger('mylogger')
handler1 = logging.FileHandler('usr.log')
handler1.setLevel(logging.INFO)
mylogger.addHandler(handler1)
handler2 = logging.FileHandler('dev.log')
handler2.setLevel(logging.ERROR)
mylogger.addHandler(handler2)
mylogger.setLevel(logging.INFO)

mylogger.critical('A critical message')
mylogger.info('An info message')

Once you've made this change, then you can use filters as David has already mentioned. Here's a quick sample filter:

class MyFilter(object):
    def __init__(self, level):
        self.__level = level

    def filter(self, logRecord):
        return logRecord.levelno <= self.__level

You can apply the filter to each of the two handlers like this:

handler1.addFilter(MyFilter(logging.INFO))
...
handler2.addFilter(MyFilter(logging.ERROR))

This will restrict each handler to only write out log messages at the level specified.

Answer 2

First: this is a rather odd thing to want to do, and strikes me as a slight misuse of the logging system. I can't imagine any situation in which it makes sense to notify the user about the normal operation of the program but not about things that are more important. The logging levels should be used to indicate importance; if you have messages that are only of interest to developers, you should be using some other mechanism to distinguish them (such as which logger you send them to).

That being said, you can implement arbitrary filtering of log records by creating a Filter subclass whose filter method implements your desired criteria and install it on the appropriate handler.