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
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.
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')
- 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.
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
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:
from .log import logger_group
import logbook logger_group = logbook.LoggerGroup() logger_group.level = logbook.WARNING
from logbook import Logger from .log import logger_group logger = Logger('yourmodule.engine') logger_group.add_logger(logger)
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
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')