Skip to content

2022-08-20 Python

Python Logging

Python Logging#

Use the source…Luke

A good place to look is the standard library’s logging module/file.

Most IDEs will go there when you cmd + click on import logging.

I was reading:

#---------------------------------------------------------------------------
#   Level related stuff
#---------------------------------------------------------------------------
#
# Default levels and level names, these can be replaced with any positive set
# of values having corresponding names. There is a pseudo-level, NOTSET, which
# is only really there as a lower limit for user-defined levels. Handlers and
# loggers are initialized with NOTSET so that they will log all messages, even
# at user-defined levels.
#

CRITICAL = 50
FATAL = CRITICAL
ERROR = 40
WARNING = 30
WARN = WARNING
INFO = 20
DEBUG = 10
NOTSET = 0

So the default logging level is 0 or NOTSET.

You can verify this yourself:

logger = logging.getLogger(__name__)
logger.level
>>>> 0

So by default all log messages should be sent…

However not all logging will print a message - only levels warning and above

Set the Logging Level from Environment Variable#

LOGLEVEL = os.environ.get('LOGLEVEL', 'WARNING').upper()
logging.basicConfig(level=LOGLEVEL)

Another way is:

logging.basicConfig(level=logging.getLevelName(log_level))

Both are fine but the top one have less calls to _nameToLevel

Logging Patterns#

The Big Tarp Pattern#

try:
    main_loop()
except Exception:
    logger.exception("Fatal error in main loop")
  • A broad catch-all
  • It can raise a number of different exceptions and you don’t want the program to terminate
  • Preferred to log and continue
  • The exception method captures the full stacktrace and logs it
  • By default, logger.exception uses the log level of ERROR.

The “Pinpoint” Pattern#

try:
    places = find_burrito_joints(criteria)
except BurritoCriteriaConflict as err:
    logger.warn("Cannot resolve conflicting burrito criteria: {}".format(err.message))
    places = list()
  • Handling a specific exception

The Message and Raise Pattern#

try:
    something()
except SomeError:
    logger.warn("...")
    raise
  • You are not actually handling the exception
  • With an existing higher level handler, you want to log that the error occurred, or the meaning of it, at a certain place in the code.
  • Most Useful in troubleshooting

The Cryptic Message Antipattern#

try:
    something()
except Exception:
    logger.error("...")

It is hard to figure out where in the code it actually broke. In that instance it is best to set exc_info

try:
    something()
except Exception:
    logger.error("something bad happened", exc_info=True)

The Most Diabolical Python Antipattern#

try:
    something()
except Exception:
    pass
  • No useful exception information is generated
  • Completely hides that there is something wrong
  • You may not even know a variable name was typed wrong - NameError is hidden

Sources#