Loguru Python Logging

Warnings

• breaking diagnose=True is the DEFAULT for all sinks. It displays variable values in exception tracebacks — leaks passwords, tokens, PII in production logs.
  Fix: Always set diagnose=False in production: logger.add('prod.log', diagnose=False)
• gotcha Default sink outputs to stderr, not stdout. Many developers expect stdout. Adding a new sink without calling logger.remove() causes duplicate log output.
  Fix: Call logger.remove() before logger.add() to remove the default stderr handler.
• gotcha Library authors must never call logger.add(). Call logger.disable(__name__) instead. Calling add() in a library forces logging config on every downstream application.
  Fix: In library code: logger.disable(__name__). In app code: logger.enable('library_name') to see library logs.
• gotcha enqueue=True (multiprocess/async safe) requires logger.complete() on shutdown. Without it, buffered messages in the queue are lost when the process exits.
  Fix: await logger.complete() in async code, or asyncio.run(logger.complete()) at shutdown.
• gotcha Passing integer log levels to logger.log() displays as anonymous level ('Level 20') instead of level name. Loguru identifies levels by name, not number.
  Fix: Use string names: logger.log('INFO', 'message') not logger.log(20, 'message')
• gotcha logger.add() returns a handler ID integer. logger.remove() requires this ID to remove a specific handler. Calling logger.remove() with no args removes ALL handlers.
  Fix: handler_id = logger.add(...); logger.remove(handler_id) to remove specific sink.

Install

• pip install loguru Python

Imports

• basic usage + reconfigure

  import sys
  from loguru import logger
  
  # Remove default stderr handler before reconfiguring
  logger.remove()
  
  # Add stdout handler with INFO level
  logger.add(
      sys.stdout,
      level='INFO',
      format='{time:YYYY-MM-DD HH:mm:ss} | {level} | {name}:{line} | {message}',
      diagnose=False,   # IMPORTANT: disable in production — prevents variable leak
      backtrace=True
  )
  
  # Add file sink with rotation
  logger.add(
      'logs/app.log',
      level='DEBUG',
      rotation='10 MB',
      retention='7 days',
      compression='zip',
      diagnose=False,
      serialize=False
  )
  
  logger.info('App started')
  logger.debug('Debug message')
  logger.warning('Watch out')
  
  # Exception catching
  try:
      1 / 0
  except Exception:
      logger.exception('Division failed')

  Default handler (id=0) outputs to stderr. Call logger.remove() or logger.remove(0) before adding sinks to avoid duplicate output. diagnose=True is the default — always set diagnose=False in production.
• library usage

  # In a library — NEVER call logger.add()
  from loguru import logger
  
  # Disable by default — let app developer enable if they want
  logger.disable(__name__)
  
  def my_library_function():
      logger.debug('Library internal log')  # no-op unless enabled by app

  Libraries must call logger.disable(__name__) and never call logger.add(). Application code enables library logging with logger.enable('library_name') if desired.

Quickstart

Loguru — reconfigure, file rotation, exception catching, structured context.

# pip install loguru
import sys
from loguru import logger

# Reconfigure: remove default stderr, add stdout + file
logger.remove()
logger.add(sys.stdout, level='INFO', diagnose=False)
logger.add('app.log', level='DEBUG', rotation='50 MB', diagnose=False)

# Basic logging
logger.debug('Debug message')
logger.info('Server started on port 8000')
logger.warning('Low disk space')
logger.error('Connection failed')
logger.critical('Database unreachable')

# Exception logging with full traceback
try:
    result = 1 / 0
except ZeroDivisionError:
    logger.exception('Calculation failed')  # logs traceback automatically

# Catch decorator
@logger.catch
def risky_function(x):
    return 100 / x

risky_function(0)  # caught and logged automatically

# Structured context with bind()
request_logger = logger.bind(request_id='req-123', user_id='usr-456')
request_logger.info('Processing payment')

# JSON output for log aggregators
logger.add('app.json', serialize=True, diagnose=False)

# Async / multiprocess safe
logger.add('async.log', enqueue=True, diagnose=False)
# On shutdown:
import asyncio
asyncio.run(logger.complete())  # flush queued messages