[Python-Dev] New-style formatting in the logging module (was Re: cpython (3.2): Issue #14123: Explicitly mention that old style % string formatting has caveats) (original) (raw)

Vinay Sajip vinay_sajip at yahoo.co.uk
Mon Feb 27 10:24:46 CET 2012

Nick Coghlan <ncoghlan gmail.com> writes:

It's half the puzzle (since composing the event fields into the actual log output is a logger action, you know the style when you supply the format string). The other half is that logging's lazy formatting currently only supporting printf-style format strings - to use brace formatting you currently have to preformat the messages, so you incur the formatting cost even if the message gets filtered out by the logging configuration.

That isn't necessarily true. Lazy formatting can work for {} and $ formatting types, not just %-formatting: see

http://plumberjack.blogspot.com/2010/10/supporting-alternative-formatting.html

Composing the event fields into the message is done by the LogRecord, which calls str() on the object passed as the format string to get the actual format string. This allows you to use any of the standard formatting schemes and still take advantage of lazy formatting, as outlined in the above post.

Although style support for Formatters is new, that's really for merging the logging event message into the overall log output (with time, severity etc.) - the support for having your own way of formatting the event message has always been there, even before str.format :-)

The Formatter style functionality is also available for 2.x through a separate logutils project which I maintain and which contains features which were added to logging in 3.2 such as QueueHandler/QueueListener:

http://pypi.python.org/pypi/logutils/

I will add a section in the logging cookbook about support for alternative formatting styles. I thought I already had, but on inspection, it appears not to be the case.

Regards,

Vinay Sajip

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