Log, don't print, in code that other code calls

print() is fine for a script you're running once. In code that other code calls, teams use logging: every message gets a level and a source, and you choose what to see without editing the code.

Before

def charge(order_id, amount):
    print("charging", order_id, amount)
    if amount <= 0:
        print("bad amount!", order_id)
        return False
    print("done", order_id)
    return True

charge("A-1001", 49.99)
charge("A-1002", 0)

charging A-1001 49.99
done A-1001
charging A-1002 0
bad amount! A-1002

Every line looks equally important, and the only way to quieten it is to delete lines.

After

import logging, sys

logging.basicConfig(stream=sys.stdout, level=logging.INFO, force=True,
                    format="%(levelname)-7s %(name)s: %(message)s")
log = logging.getLogger("orders")

def charge(order_id, amount):
    log.debug("charging %s for %.2f", order_id, amount)
    if amount <= 0:
        log.warning("order %s has amount %.2f, skipped", order_id, amount)
        return False
    log.info("charged order %s", order_id)
    return True

charge("A-1001", 49.99)
charge("A-1002", 0)

INFO    orders: charged order A-1001
WARNING orders: order A-1002 has amount 0.00, skipped

The debug line is still in the code, just hidden. Change level=logging.INFO to logging.DEBUG and run it again: nothing else changes and the detail comes back.

Why teams do this

• Levels. debug for you, info for the normal story, warning and error for the things someone should look at.
• Sources. getLogger("orders") names where a message came from, so one noisy module can be turned down on its own.
• Destinations. The same calls can write to a file, a terminal or a log service, chosen in one place at start-up.
• Cheap when off. log.debug("... %s", x) only builds the string if debug is on. An f-string is built every time.

When not to use it

Output that is the program's job (a report, a table, the answer a command-line tool prints) belongs on print(). Logging is for messages about the program, not from it.