How to use Python’s RotatingFileHandler

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Python provides 5 logging levels out of the box (in increasing order of severity): DEBUG, INFO, WARNING, ERROR and CRITICAL. The default one is WARNING. The docs says, that

Logging messages which are less severe than lvl will be ignored.

So if you use .debug with the default settings, you won’t see anything in your logs.

The easiest fix would be to use logger.warning function rather than logger.debug:

import logging
from logging.handlers import RotatingFileHandler

logger = logging.getLogger('my_logger')
handler = RotatingFileHandler('my_log.log', maxBytes=2000, backupCount=10)
logger.addHandler(handler)

for _ in range(10000):
    logger.warning('Hello, world!')

And if you want to change logger level you can use .setLevel method:

import logging
from logging.handlers import RotatingFileHandler

logger = logging.getLogger('my_logger')
logger.setLevel(logging.DEBUG)
handler = RotatingFileHandler('my_log.log', maxBytes=2000, backupCount=10)
logger.addHandler(handler)

for _ in range(10000):
    logger.debug('Hello, world!')

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