Python logging

Code

import logging

logger = logging.getLogger(__name__)
logger.info("My message")

Error levels

Info levels - INFO

• INFO: general info
• WARNING: Minor problem
  • still info, give more details when debugging
• ERROR: Major problem
  • something's broken! high priority
• CRITICAL: Critical problem
  • system as as whole is down

warnings module

Fullchee Values

warns devs, not end users

Using warnings module

Useful for deprecation notices

stacklevel should be 2+ - default stacklevel is 1 - not helpful, doesn't say who called the module

import warnings

def old_func():
    warnings.warn(
        DeprecationWarning('use new_func instead'),
        stacklevel=2
    )

$ PYTHONWARNINGS=once python3 file.py

$ python3 -Wonce file.py

$ python3 -Werror file.py

logging.warning vs warnings.warn

• logging.warning
  • issue with input/user
  • nothing the client app can do
• warnings.warn
  • dev issue
    • examples
      • deprecated code
      • abstract class not implement

Adding a traceback

log stack trace

can only be in an except clause

• otherwise you get an error

except Exception as e:
    logger.exception("Exception message")

Shorthand for

logger.error("something weird happened", exc_info=True)

• exc_info=True is useful if you want to log an error

Deprecated warning

warnings.warn(
              "message",
              category=DeprecationWarning,
)

Tips on logging message

• multiple machines?
  • include a request ID so all logs can be grouped
• per user
  • easily debug a user specific issue

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Last update: 2023-04-24