RotatingFileHandler

RotatingFileHandler is a log handler in Python's standard logging library, provided by the logging.handlers module. It writes log records to a file and rotates the file when its size reaches a configured threshold, enabling continuous logging without unbounded file growth.

The handler is configured by parameters such as filename, mode, maxBytes, backupCount, encoding, and delay. filename is the path to the log file, mode defaults to append ('a'), maxBytes sets the rotation threshold, backupCount determines how many rotated files to keep, encoding specifies the file encoding, and delay defers file creation until the first log record is emitted. If maxBytes is less than or equal to zero, rotation is disabled. If backupCount is zero, no rotation backups are kept.

Rotation works as log records are emitted. When the current file length exceeds maxBytes, doRollover is invoked. The existing log files are renamed to increment their suffix numbers (for example, filename.1, filename.2, and so on), up to the limit defined by backupCount. The latest log file is then created anew with the original filename to continue logging. Older backups beyond backupCount are removed, preventing uncontrolled growth of log files.

File naming typically uses suffixes appended to the base filename, such as .1, .2, etc. This makes it possible to retain a fixed number of recent logs while discarding the oldest ones as new ones are created.

RotatingFileHandler is suitable for size-based log management within single-process or simple multi-threaded environments. For multi-process scenarios or time-based rotation, other handlers like TimedRotatingFileHandler or alternative logging configurations may be preferred.