How to specify levels in python logging module?

Question

I need two handlers. One for file logging and another one for stream logging in the console. I need to specify levels for each handler. Notice that my levels are going to be something like the following ones.

Stream Handler --> INFO

File Handler --> WARNING, ERROR, CRITICAL

Here is my code.

    # Create a custom logger
    logger = logging.getLogger('DBMQ')
    logger.setLevel(logging.DEBUG)

    stream_handler = logging.StreamHandler()
    stream_handler.setLevel(logging.INFO)

    logger.addHandler(stream_handler)

    file_handler = logging.FileHandler('./data/file.log')
    file_handler.setLevel(logging.WARNING)

    logger.addHandler(file_handler)

Now, it has to operate like this:

logger.debug('hey') # Nothing should happen
logger.info('hey') # only stream handler should log this message
logger.warning('hey') # only file handler should log this message but the stream does too
logger.error('hey') # only file handler should log this message but the stream does too
logger.critical('hey') # only file handler should log this message but the stream does too

I need to disallow the stream handler from logging the warning, error, and critical logging levels. Is there any way to filter this handler?

The file handler works well, but the stream handler logs those last three levels. I just want it to log the `INFO` level only. — Sadra, Oct 09 '20 at 12:48
Log levels are always minimum levels. INFO means "log INFO or higher". Scenarios where you want a log to contain some logs, but not some more important logs are hard to construct. — user2390182, Oct 09 '20 at 12:54
@schwobaseggl exactly. I'm looking for a way to log a certain level, but as you mentioned, It does a level or higher..!! — Sadra, Oct 09 '20 at 12:58

score 4 · Accepted Answer · answered Oct 09 '20 at 13:00

4

You could create your own Handler:

logger = logging.getLogger('DBMQ')
logger.setLevel(logging.DEBUG)


class MyStreamHandler(logging.StreamHandler):
    def emit(self, record):
        if record.levelno == self.level:
            super().emit(record)

stream_handler = MyStreamHandler()
stream_handler.setLevel(logging.INFO)

logger.addHandler(stream_handler)

file_handler = logging.FileHandler('./data/file.log')
file_handler.setLevel(logging.WARNING)

logger.addHandler(file_handler)

answered Oct 09 '20 at 13:00

Lufftre

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Thanks. It works properly, but the stream handler still logs a message with `warning` level. Now, both error and critical are disabled for Stream handler which is good. – Sadra Oct 09 '20 at 13:18
It works as you described for me. Did you change anything from my example? – Lufftre Oct 09 '20 at 13:29
I just added my File handler. Tried again and works well. Thanks. – Sadra Oct 09 '20 at 13:30
Is it a common way? Is there any way to add this class to a `.conf` file? I want to write up a configuration file and use `fileConfig()` method. – Sadra Oct 09 '20 at 21:43

score 0 · Answer 2 · answered Oct 09 '20 at 13:03

The "level" system does not work to as disable/enable single "levels". https://docs.python.org/3/library/logging.html#logging-levels

The "level" is a single integer value. If the message's level is over logger/handler level, it will be showed. Just that simple.

If you need another logic the only way is implement you own handlers.

score 0 · Answer 3 · answered Mar 05 '23 at 23:49

I liked Lufftre's answer here, but I don't like the idea of creating a whole class just for this, so I'm basically patching the StreamHandler's emit function into a lambda function, like so:

chs = logging.StreamHandler()
chs.setLevel(logging.INFO)
chs.semit = chs.emit # You have to "clone" original emit, as there is no super()
chs.emit = lambda record: chs.semit(record) if record.levelno == chs.level else None
logger.addHandler(chs)

I think it looks cleaner. If you want multiple handlers working in this way, then you can first define a function that is this lambda and then patch the emit with that function object.

# Set up new emit
nemit = lambda record, sh: sh.semit(record) if record.levelno == sh.level else None

# Create first handler
ch = logging.StreamHandler()
ch.setLevel(logging.ERROR)
ch.semit = ch.emit 
ch.emit = lambda record: nemit(record, ch)

# Create second handler
chs = logging.StreamHandler()
chs.setLevel(logging.INFO)
chs.semit = chs.emit
chs.emit = lambda record: nemit(record, chs)

logger.addHandler(ch)
logger.addHandler(chs)

How to specify levels in python logging module?

3 Answers3

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