Python logging and rotating files

Question

I have a python program that is writing to a log file that is being rotated by Linux's logrotate command. When this happens I need to signal my program to stop writing to the old file and start writing to the new one. I can handle the signal but how do I tell python to write to the new file?

I am opening the file like this:

logging.basicConfig(format='%(asctime)s:%(filename)s:%(levelname)s:%(message)s',filename=log_file, level=logging.INFO)

and writing to it like this:

logging.log(level,"%s" % (msg))

The logging modules look very powerful but also overwhelming. Thanks.

Regretful

Answer 1

Don't use logging.basicConfig, use WatchedFileHandler. Here's how to use it.

import time
import logging
import logging.handlers

def log_setup():
    log_handler = logging.handlers.WatchedFileHandler('my.log')
    formatter = logging.Formatter(
        '%(asctime)s program_name [%(process)d]: %(message)s',
        '%b %d %H:%M:%S')
    formatter.converter = time.gmtime  # if you want UTC time
    log_handler.setFormatter(formatter)
    logger = logging.getLogger()
    logger.addHandler(log_handler)
    logger.setLevel(logging.DEBUG)

log_setup()
logging.info('Hello, World!')
import os
os.rename('my.log', 'my.log-old')
logging.info('Hello, New World!')

Hedve

@TyHitzeman You mean RotatingFileHandler. RollingFileHandler is a Java thing. – Hedve

This works great for the OP and other Unix users. FYI for Windows users: you can't use WatchedFileHandler according to the docs. Checkout RotatingFileHandler instead – Espouse

Answer 2

You may want to look at WatchedFileHandler to implement this, or as an alternative, implement log rotation with RotatingFileHandler, both of which are in the logging.handlers module.

Milliary

Thanks! If I need to I can go this route but currently the file is already being rotated outside of my code. I was hoping there was a way to close the log and reopen it. – Regretful

Ah ... WatchedFileHandler ... now I get it. – Regretful

is there an option to have the rolled logs not currently being written to compressed (zipped)? E.g the docs mention that RotatingFileHandler would produce app.log.1, app.log.2, ... but ideally these rolled logs would be optionally compressed to save space. – Pons

Answer 3

from logging import handlers

handler = handlers.TimedRotatingFileHandler(filename, when=LOG_ROTATE)

handler.setFormatter(logging.Formatter(log_format, datefmt="%d-%m-%Y %H:%M:%S"))

#LOG_ROTATE = midnight    
#set your log format

This should help you in handling rotating log

Singlebreasted

Answer 4

Since rotation is already being done by logrotate, in your signal handler you should just call logging.basicConfig(...) again and that should reopen the log file.

Sheepherder

basicConfig is a no-op if it's already been run. To force it to reconfigure everything again, you have to set logging.root.handlers = [] first. – Coreycorf