Logbook in Libraries

Logging becomes more useful the higher the number of components in a system that are using it. Logbook itself is not a widely supported library so far, but a handful of libraries are using the logging already which can be redirected to Logbook if necessary.

Logbook itself is easier to support for libraries than logging because it does away with the central logger registry and can easily be mocked in case the library is not available.

Mocking Logbook

If you want to support Logbook in your library but not depend on it you can copy/paste the following piece of code. It will attempt to import logbook and create a Logger and if it fails provide a class that just swallows all calls:

try:
    from logbook import Logger
except ImportError:
    class Logger(object):
        def __init__(self, name, level=0):
            self.name = name
            self.level = level
        debug = info = warn = warning = notice = error = exception = \
            critical = log = lambda *a, **kw: None

log = Logger('My library')

Best Practices

• A library that wants to log to the Logbook system should generally be designed to provide an interface to the record dispatchers it is using. That does not have to be a reference to the record dispatcher itself, it is perfectly fine if there is a toggle to switch it on or off.

• The channel name should be readable and descriptive.

• For example, if you are a database library that wants to use the logging system to log all SQL statements issued in debug mode, you can enable and disable your record dispatcher based on that debug flag.

• Libraries should never set up log setups except temporarily on a per-thread basis if it never changes the stack for a longer duration than a function call in a library. For example, hooking in a null handler for a call to a noisy function is fine, changing the global stack in a function and not reverting it at the end of the function is bad.

Example Setup

Consider how your logger should be configured by default. Users familiar with logging from the standard library probably expect your logger to be disabled by default:

import yourmodule
import logbook

yourmodule.logger.enable()

def main():
    ...
    yourmodule.something()
    ...

if __name__ == '__main__':
    with logbook.StderrHandler():
        main()

or set to a high level (e.g. WARNING) by default, allowing them to opt in to more detail if desired:

import yourmodule
import logbook

yourmodule.logger.level = logbook.WARNING

def main():
    ...
    yourmodule.something()
    ...

if __name__ == '__main__':
    with logbook.StderrHandler():
        main()

Either way, make sure to document how your users can enable your logger, including basic use of logbook handlers. Some users may want to continue using logging, so you may want to link to LoggingHandler.

Multiple Logger Example Setup

You may want to use multiple loggers in your library. It may be worthwhile to add a logger group to allow the level or disabled attributes of all your loggers to be set at once.

For example, your library might look something like this:

yourmodule/__init__.py

 from .log import logger_group

yourmodule/log.py

import logbook

logger_group = logbook.LoggerGroup()
logger_group.level = logbook.WARNING

yourmodule/engine.py

from logbook import Logger
from .log import logger_group

logger = Logger('yourmodule.engine')
logger_group.add_logger(logger)

yourmodule/parser.py

from logbook import Logger
from .log import logger_group

logger = Logger('yourmodule.parser')
logger_group.add_logger(logger)

The library user can then choose what level of logging they would like from your library:

import logbook
import yourmodule

yourmodule.logger_group.level = logbook.INFO

They might only want to see debug messages from one of the loggers:

import logbook
import yourmodule

yourmodule.engine.logger.level = logbook.DEBUG

Debug Loggers

Sometimes you want to have loggers in place that are only really good for debugging. For example you might have a library that does a lot of server/client communication and for debugging purposes it would be nice if you can enable/disable that log output as necessary.

In that case it makes sense to create a logger and disable that by default and give people a way to get hold of the logger to flip the flag. Additionally you can override the disabled flag to automatically set it based on another value:

class MyLogger(Logger):
    @property
    def disabled(self):
        return not database_connection.debug
database_connection.logger = MyLogger('mylibrary.dbconnection')