logging — Logging Facility

import logging

Basic Usage

import logging

logging.basicConfig(level=logging.DEBUG)

logging.debug("Debug message")      # Detailed diagnostic info
logging.info("Info message")        # Confirmation things work
logging.warning("Warning message")  # Something unexpected
logging.error("Error message")      # Serious problem
logging.critical("Critical!")       # Program may crash

Configuring Output

import logging

logging.basicConfig(
    level=logging.INFO,
    format='%(asctime)s - %(name)s - %(levelname)s - %(message)s',
    datefmt='%Y-%m-%d %H:%M:%S',
    filename='app.log',  # Log to file (omit for console)
    filemode='a'         # 'a' = append, 'w' = overwrite
)

Named Loggers

import logging

logger = logging.getLogger(__name__)
logger.setLevel(logging.DEBUG)

# Console handler
console = logging.StreamHandler()
console.setLevel(logging.INFO)
console.setFormatter(logging.Formatter('%(name)s - %(levelname)s - %(message)s'))
logger.addHandler(console)

# File handler
file_handler = logging.FileHandler('debug.log')
file_handler.setLevel(logging.DEBUG)
file_handler.setFormatter(logging.Formatter('%(asctime)s - %(message)s'))
logger.addHandler(file_handler)

logger.info("This goes to console AND file")
logger.debug("This goes to file only")

Log Levels

Exception Logging

import logging

try:
    result = 1 / 0
except ZeroDivisionError:
    logging.error("Division failed", exc_info=True)
    # or
    logging.exception("Division failed")  # Same, always includes traceback

Common Pitfalls

• basicConfig only works once — call it before any logging
• Root logger vs named loggers — use getLogger(__name__) in modules
• Don't use print() for logging — use logging module for production code

Official Documentation

logging — Logging facility for Python

API Reference

Configuration

Log Levels Methods

Handlers