The basics of the logging module mystify me

[Thomas Jollans]

On 2018-04-19 03:00, Skip Montanaro wrote:
> This session is from Python 3.6.5 on Linux:
> 
>>>> import logging
>>>> log = logging.getLogger()
>>>> log.level
> 30
>>>> logging.WARN
> 30
>>>> log.warn("Awk! Goodbye...")
> Awk! Goodbye...
>>>> log.level = logging.INFO
>>>> log.info("Awk! Goodbye...")
>>>> log.level
> 20
>>>> log.level == logging.INFO
> True
>>>> log.setLevel(logging.INFO)
>>>> log.info("Awk! Goodbye...")
>>>> log.isEnabledFor(logging.INFO)
> True
> 
> Why do the two log.info(...) calls not produce output on stderr when
> the level has clearly been set to logging.INFO? There is an active
> stream handler as demonstrated by the successful log.warn(...) call.

Loggers have levels, and log handlers have levels. To get log messages
to display, you have to add a log handler (with the right level) to your
logger.

Now, you never added a handler, so why are the warnings printing? Log
messages that have nowhere to go are given to the "last resort" handler,
logging.lastResort
<https://docs.python.org/3/library/logging.html#logging.lastResort>,
which has its level set to WARNING.

You *could* change this:

Python 3.6.3 |Anaconda, Inc.| (default, Oct 13 2017, 12:02:49)
[GCC 7.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import logging
>>> log = logging.getLogger()
>>> log.setLevel(logging.INFO)
>>> log.warn('warning 1')
warning 1
>>> log.info('info 1')
>>> logging.lastResort
<_StderrHandler <stderr> (WARNING)>
>>> logging.lastResort.setLevel(logging.INFO) # DON'T DO THIS THOUGH
>>> log.info('info 2')
info 2
>>>

Of course you should rather be creating your own handler

>>> handler = logging.StreamHandler()
>>> handler.setLevel(logging.DEBUG)
>>> log.addHandler(handler)
>>> log.setLevel(logging.DEBUG)
>>> log.debug('test test test')
test test test
>>>

Or, more often than not, it's best to use the logging module's
configuration system that creates the right web of handlers and
formatters for you.


-- Thomas

> 
> I really don't like the logging module, but it looks like I'm stuck
> with it. Why aren't simple/obvious things either simple or obvious?