Log Levels in Practice

• Python defines five standard levels with increasing severity:
  • DEBUG (10): Detailed diagnostic information.
  • INFO (20): Confirmation that things are working normally.
  • WARNING (30): An indication of potential problems or deprecation.
  • ERROR (40): A failure in a specific operation.
  • CRITICAL (50): A serious error causing program termination.
• NOTSET (0) causes a logger to inherit its parent’s effective level.
• Appropriate use of these levels lets you adjust verbosity without changing code.

Two-Stage Filtering: Logger vs Handler

• Logger Level: First gate: records below logger.level are discarded immediately.
• Handler Level: Second gate: each handler only emits records at or above its handler.level.
• This allows, for example, DEBUG messages to be logged to a file but only WARNING and above to the console.

Configuring Logger & Handlers

• Use logger.setLevel(...) to control which messages the logger accepts.
• Use handler.setLevel(...) to control which accepted messages each handler emits.
• Attach multiple handlers for different ouputs (e.g., console vs file) with independent levels.

# Log levels in practice

import logging
import sys

print("Log levels in practice")
print("------\n")

for lvl in (
    logging.DEBUG,
    logging.INFO,
    logging.WARNING,
    logging.ERROR,
    logging.CRITICAL,
):
    print(
        f"{logging.getLevelName(lvl):8} = {lvl}"
    )

# Two-stage filtering

print("\n")
print("Two stage filtering")
print("------\n")

filter_logger = logging.getLogger("demo.filter")
filter_logger.setLevel(logging.INFO)

stream_handler = logging.StreamHandler(sys.stdout)
stream_handler.setLevel(logging.ERROR)

filter_logger.addHandler(stream_handler)

filter_logger.info("INFO: will not be shown")
filter_logger.error("ERROR: will be shown")

# Configuring logs and handlers

print("\n")
print("Configuring logs and handlers")
print("------\n")

data_logger = logging.getLogger("demo.data")
data_logger.setLevel(logging.DEBUG)

data_sh = logging.StreamHandler(sys.stdout)
data_sh.setLevel(logging.ERROR)

data_fh = logging.FileHandler("process.log", "w")
data_fh.setLevel(logging.INFO)

data_logger.addHandler(data_sh)
data_logger.addHandler(data_fh)

data_logger.debug("DEBUG: will be dropped")
data_logger.info("INFO: file only")
data_logger.warning("WARNING: file only")
data_logger.error("ERROR: file and console")
data_logger.critical("CRITICAL: file and console")