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Why should you use the logging module instead of print() for diagnostic output? What does logging.basicConfig() do, and what are its common pitfalls? How do you format log output to include the timestamp, logger name, level, and message? What is the relationship between Logger, Handler, Formatter, and Filter in the logging module? Why is it recommended to use logging.getLogger(__name__) instead of the root logger in library and module code? How do you configure logging using a dictionary via logging.config.dictConfig? How do you include structured context (like a request ID) in every log message using LoggerAdapter or extra? What's the difference between propagate=True and propagate=False on a logger? What's the danger of using %-style, f-string, or .format() interpolation directly in a log call's message argument versus lazy % substitution? How do RotatingFileHandler and TimedRotatingFileHandler prevent log files from growing unbounded?
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Python › Logging

What are Python's five standard logging severity levels, from lowest to highest?

Beginner

Answer

DEBUG (detailed diagnostic info), INFO (confirmation things work as expected), WARNING (something unexpected but not fatal), ERROR (a serious problem, functionality failed), and CRITICAL (a very serious error, program may be unable to continue). Each level has a numeric value, and a logger only emits messages at or above its configured threshold.
import logging
logging.basicConfig(level=logging.WARNING)
logging.debug('not shown')
logging.info('not shown')
logging.warning('shown')  # WARNING:root:shown
logging.error('shown')
Real-world example A deployment sets the root logger to WARNING in production to reduce noise, but switches to DEBUG temporarily when investigating a reported bug.

Common follow-ups: How do numeric level values compare (e.g., logging.INFO value)?;How do you set different levels per module?

Logging;Debugging & Profiling
All 15 Logging questions