Logging Instead of print

Why this matters

print is fine for a learner’s first program. Real programs use logging: messages come with a severity level, can be switched on or off without editing code, and can go to the screen, a file or a service.

The idea

import logging

logging.basicConfig(level=logging.INFO, format="%(levelname)s: %(message)s")
log = logging.getLogger("shop")

log.debug("cart contents: [...]")      # hidden: below INFO
log.info("order 7 accepted")           # INFO: order 7 accepted
log.warning("stock is low")            # WARNING: stock is low
log.error("payment failed")            # ERROR: payment failed

Levels, from quiet to loud: DEBUG, INFO, WARNING, ERROR, CRITICAL. A logger shows messages at or above its level.

Under the hood a logger creates messages, a handler decides where they go (StreamHandler(stream) writes to a stream) and a formatter decides how they look. Inside an except block, log.exception("...") also records the traceback.

Try it

Change level=logging.INFO to logging.DEBUG, then to logging.ERROR. Which messages appear each time?

Your turn

Write three functions:

• make_logger(name, stream): return the logger called name, at level INFO, with one handler that writes to stream in the format LEVEL: message ("%(levelname)s: %(message)s"). Set logger.propagate = False so the root logger does not print it a second time. Calling the function again with the same name must replace the old handler, not add a second one.
• process_orders(orders, logger): each order is a dictionary like {"id": 3, "qty": 2}. A quantity of 0 or less logs a warning order 3: bad quantity 0; otherwise log an info message order 3: ok. Return how many orders were fine.
• safe_parse(text, logger): return int(text); if the text is not a whole number, log an error could not parse 'abc' (with !r) and return None.

Recap

• Use logging levels instead of print so output can be filtered and redirected.
• A logger makes messages, a handler picks the destination, a formatter picks the look.
• log.exception(...) inside except records the traceback too.

Common mistakes

• Doing nothing and expecting INFO to appear. The default level is WARNING; set a level.
• Adding a handler every time a function runs, so each message prints several times.
• Logging secrets such as passwords or tokens.