[Python-Dev] summary of transitioning from % to {} formatting (original) (raw)

Vinay Sajip vinay_sajip at yahoo.co.uk
Sun Oct 4 00:43:18 CEST 2009

Brett Cannon <brett python.org> writes:

No one is saying we should deprecate % any time soon on strings themselves or anywhere. This discussion is purely in regards to argparse and logging to transition their APIs over to {} formatting which would most likely involve some deprecation for using % formatting in those APIs. But % formatting on strings themselves is not directly being discussed here.

While I have no problem with supporting {}-formatting in logging if we find a good way of doing it, one thing that bothers me about transitioning the logging API to deprecate or otherwise de-emphasise %-formatting is the question of performance. Now, str.format is more flexible than str.mod and so some performance loss may be tolerable in many scenarios. However, in the feedback I regularly get about logging, people are concerned about performance. No-one ever comes up with any hard numbers, but logging is often bashed as slow (see e.g. Andrii Mishkovskyi's LoggingPackage page on the Python wiki). I don't especially want to add fuel to the fire, as any performance degradation caused by supporting {}-formatting will likely just result in more finger-pointing at logging in general.

Regards,

Vinay Sajip

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