Standard Library

Logging

logging records operational events without using print as infrastructure.

A logger name records which part of the program produced the event. The handler and formatter choose where and how the event is shown.

Source

import logging
import sys

logger = logging.getLogger("example.worker")
logger.setLevel(logging.DEBUG)
handler = logging.StreamHandler(sys.stdout)
handler.setLevel(logging.INFO)
parts = ["%(levelname)s", "%(name)s", "%(message)s"]
formatter = logging.Formatter(":".join(parts))
handler.setFormatter(formatter)
logger.handlers[:] = [handler]
logger.propagate = False

logger.debug("hidden detail")
logger.info("service started")
logger.warning("disk almost full")

Output

INFO:example.worker:service started
WARNING:example.worker:disk almost full

Levels are thresholds. Raising the handler level to WARNING suppresses later INFO records without changing the call sites.

Source

import logging
import sys

logger = logging.getLogger("example.worker")
logger.setLevel(logging.DEBUG)
handler = logging.StreamHandler(sys.stdout)
handler.setLevel(logging.WARNING)
parts = ["%(levelname)s", "%(name)s", "%(message)s"]
formatter = logging.Formatter(":".join(parts))
handler.setFormatter(formatter)
logger.handlers[:] = [handler]
logger.propagate = False

logger.info("hidden after threshold change")
logger.error("write failed")

Output

ERROR:example.worker:write failed

Notes

Configure logging once; call named loggers throughout the program.
Logger and handler levels both participate in filtering.
Use exceptions for control flow failures, logging for operational evidence, and warnings for soft compatibility problems.

Run the complete example

Expected output

INFO:example.worker:service started
WARNING:example.worker:disk almost full
ERROR:example.worker:write failed

Execution time appears here after you run the example.

Logging

See also

Run the complete example

Example code

Expected output