Why does Python RotatingFileHandler's "maxBytes " not work

Question

I'm trying to do a test run of the logging module's RotatingFileHandler as follows:

from logging import getLogger, Formatter
from logging.handlers import RotatingFileHandler

MAX_LOG_SIZE = 2000

_null = lambda *s: None
LOG_FORMAT = '%(process)d [%(name)s %(asctime)s] %(levelname)s: %(message)s'

class Logger(object):
    __slots__ = '_Logger__logger'

    def __init__(self, name='main'):
        self.__logger = getLogger(name)
        if not self.__logger.handlers:
            self.add_handler(name)

    def add_handler(self, name):

        file_name = 'log.log'
        handler = RotatingFileHandler(file_name, 'a+', MAX_LOG_SIZE)
        handler.setFormatter(Formatter(LOG_FORMAT))
        self.__logger.addHandler(handler)
        self.__logger._file_name = file_name

    def ERROR(self, msg, *args):
        self.__logger.error(msg, *args, **{})

if __name__ == '__main__':
    logger = Logger()
    for i in range(1000):
        logger.ERROR('logger.content')

However, with MAX_LOG_SIZE = 2000, the resulting of log.log file contains too much data large than 2000 bytes

How can I limit max size of the logfile?

Answer 1

You need to read the documentation more carefully: you are missing the kwargs maxBytes and backupCount.

Replace this

handler = RotatingFileHandler(file_name, 'a+', MAX_LOG_SIZE)

for this

handler = RotatingFileHandler(file_name, 'a+', maxBytes=MAX_LOG_SIZE, backupCount=5)

Notice you shall set a backupCount value that fits your needs, I just used a random one.

A further explanation of why your piece of code does not roll the file is because the value backupCount is 0. See the following:

You can use the maxBytes and backupCount values to allow the file to rollover at a predetermined size. When the size is about to be exceeded, the file is closed and a new file is silently opened for output. Rollover occurs whenever the current log file is nearly maxBytes in length; but if either of maxBytes or backupCount is zero, rollover never occurs, so you generally want to set backupCount to at least 1, and have a non-zero maxBytes.