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Why should you use the logging module instead of print() for diagnostic output? What are Python's five standard logging severity levels, from lowest to highest? What does logging.basicConfig() do, and what are its common pitfalls? 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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How do you format log output to include the timestamp, logger name, level, and message?

Beginner

Answer

A Formatter string uses LogRecord attribute placeholders like %(asctime)s, %(name)s, %(levelname)s, and %(message)s, combined into a single format string applied to every emitted record by a handler's setFormatter() call.
import logging

formatter = logging.Formatter('%(asctime)s [%(name)s] %(levelname)s: %(message)s')
handler = logging.StreamHandler()
handler.setFormatter(formatter)
logging.getLogger().addHandler(handler)
logging.warning('Low disk space')
# 2024-01-15 10:30:00,123 [root] WARNING: Low disk space
Real-world example An ops team standardizes on a JSON-formatted log line (using a custom Formatter subclass) across all services so their log aggregation platform can parse fields consistently.

Common follow-ups: What other LogRecord attributes are commonly used (%(module)s, %(lineno)d)?;How would you write a JSON-output Formatter?

Logging;Debugging & Profiling
All 15 Logging questions