Logging to Files

Basic File Logging with FileHandler

• Use logging.FileHandler to write log records to a file.
• mode='a' (append) preserves existing logs; mode='w' (write) overwrites on each run.
• You can specify encoding (e.g., 'utf-8') and delay=True to open the file only on first write.

Size-Based Rotation with RotatingFileHandler

• RotatingFileHandler rotates when the file reaches maxBytes.
• backupCount determines how many old files to keep (.1, .2, …).
• New rotations rename existing backups, deleting the oldest beyond backupCount.

Time-Based Rotation with TimedRotatingFileHandler

• TimedRotatingFileHandler rotates based on elapsed time (when, interval).
• Common when values (case insensitive): 'S', 'M', 'H', 'D', 'midnight', 'W0'-'W6'
  • 'S' – Rotate every N seconds (as given by interval), useful for very short-lived scripts or testing.
  • 'M' – Rotate every N minutes, good for high-volume services where hourly isn’t fine-grained enough.
  • 'H' – Rotate every N hours, often used for long-running daemons that batch logs hourly.
  • 'D' – Rotate every N days, for simple daily log files without tying to midnight.
  • 'midnight' – Rotate once per day exactly at midnight (local time), regardless of interval, ideal for calendar-aligned logs.
  • 'W0'–'W6' – Rotate weekly on a specific weekday, where W0 = Monday through W6 = Sunday. Use interval weeks between rotations.
• backupCount limits number of rotated files; use .suffix to customize timestamp format.

import logging
import logging.handlers
import os
import time

def cleanup_log_files(base_name: str):
    for file_name in os.listdir("."):
        if file_name.startswith(base_name):
            os.remove(file_name)

# Basic logging with FileHandler
print("Basic logging with FileHandler")
print("-------\n")

basic_logger = logging.getLogger("file.basic")
basic_logger.setLevel(logging.DEBUG)

basic_fh = logging.FileHandler(
    "basicfile.log", delay=True, encoding="utf-8"
)
basic_fh.setLevel(logging.INFO)

basic_logger.addHandler(basic_fh)

basic_logger.info("INFO: will be written to file")

# Size-based log rotation with RotatingFileHandler
print("Size-based log rotation with RotatingFileHandler")
print("-------\n")

rotating_logs_filename = "rotatingfile.log"

cleanup_log_files(rotating_logs_filename)

rotating_logger = logging.getLogger("file.rotating")
rotating_logger.setLevel(logging.DEBUG)

rotating_fh = logging.handlers.RotatingFileHandler(
    rotating_logs_filename,
    maxBytes=500,
    backupCount=2,
    encoding="utf-8",
)
rotating_fh.setFormatter(
    logging.Formatter("%(levelname)-8s %(message)s")
)

rotating_logger.addHandler(rotating_fh)

for i in range(30):
    rotating_logger.info(f"Entry {i}: {'Z' * 50}")
    time.sleep(0.05)

# Time-based log rotation with TimedRotatingFileHandler
print("Time-based log rotation with TimedRotatingFileHandler")
print("-------\n")

timed_rotating_logs_filename = "timedrotatingfile.log"

cleanup_log_files(timed_rotating_logs_filename)

timed_rotating_logger = logging.getLogger("file.timed")
timed_rotating_logger.setLevel(logging.DEBUG)

timed_rotating_fh = logging.handlers.TimedRotatingFileHandler(
    timed_rotating_logs_filename,
    when="s",
    interval=3,
    backupCount=2,
    encoding="utf-8",
)
timed_rotating_fh.setFormatter(
    logging.Formatter("%(levelname)-8s %(message)s")
)

timed_rotating_logger.addHandler(timed_rotating_fh)

for i in range(30):
    timed_rotating_logger.info(f"Entry {i}: {'Z' * 50}")
    time.sleep(0.5)