How to configure all loggers in an application

Question

Python\'s logging module lets modules or classes define their own loggers. And different loggers can have different handlers. Some of them may choose to log to a file, while som

Answer 1

You should probably look into the Python Logging HOWTO to understand how it works.

In short, all that modules usually do is getting a logger of the form G_LOG = logging.getLogger('package.name') and sending messages to the logger: G_LOG.info('some message'), G_LOG.exception('something bad happened'). Modules should not usually configure anything.

The application that uses the modules can turn the logging on and configure the handlers based on the logger names:

• listen all messages, or
• listen only messages above a certain threshold, or
• listen messages only from loggers whose name starts with package, or
• listen messages only from loggers whose name starts woth package.name, etc

The easiest way is to configure logging through logging.basicConfig somewhere in the beginning of your application:

logging.basicConfig(level=logging.DEBUG,
                    format='%(asctime)s %(levelname)-8s %(message)s',
                    datefmt='%Y-%m-%d %H:%M:%S',
                    filename=log_file, filemode='a')

That way you will write all logging messages from all modules to the log_file.

If you need a more detailed logging strategy (put logs from different loggers to different files, or send stacktraces to a separate file), it is better to define a logging config file and configure logging using logging.config.dictConfig or logging.config.fileConfig.

P.S. I usually create two loggers as module variables:

G_LOG = logging.getLogger(__name__)
ST_LOG = logging.getLogger('stacktrace.' + __name__)

to G_LOG I send only one-line messages. To ST_LOG I duplicate important messages using ST_LOG.exception which implicitly has exc_info=True and writes the stacktrace of the current exception.

At the start of the application I load a configuration that configures two loggers (and two file handlers for them): one that receives messages that start with stacktrace and has propagate=0 (that is stacktrace messages are not visible at the top) and the root logger that handles the rest of the messages. I will not put here my complete log config files, since it is a useful home work to understand how it all works.