[Python-Dev] Using logging in the stdlib and its unit tests (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Sat Dec 11 10:07:55 CET 2010

On Sat, Dec 11, 2010 at 3:06 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

However, the confusion that this setup will engender is that encountered by Bill: by default, info() messages are silenced rather than displayed on stdout.

Notice that even the recommended "basicConfig" approach to resolving this is subtly flawed, since your choices are to either display info messages on stderr (by specifying just a level argument) or to redirect warning() et al to stdout instead of stderr (by also specifying a stream argument). The "correct" answer (info and below to stdout, warning and above to stderr) is actually quite tricky to set up, since handlers have no innate concept of a "maximum level", so you have to provide a filter function (more on that below [1]).

I need to correct this comment, since it's wrong and basicConfig(level=logging.INFO) actually does the right thing.

As Glenn mentioned later in the thread, the output of logging.info and logging.debug messages is distinct from an application's normal operational output that is emitted on stdout. So making it easy to emit such messages on stderr is the right thing to do - it's OK that it requires a bit of additional set up to get them onto stdout instead (since they really don't belong there most of the time).

I know my own rule of thumb is going to be updated along the lines of "Am I writing to sys.stderr, or guarding output with a verbosity flag? Then perhaps I should be using the logging module rather than print statements".

Cheers, Nick.

-- Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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