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

Nick Coghlan ncoghlan at gmail.com
Thu Oct 1 23:54:20 CEST 2009

Vinay Sajip wrote:

So there's no need to change modules like logging to explicitly provide support for {}-formatting? What's not to like? ;-) Something like this perhaps should have been added in at the same time as str.format went in.

I believe classes like fmt_braces/fmt_dollar/fmt_percent will be part of a solution, but they aren't a complete solution on their own. (Naming the three major string formatting techniques by the key symbols involved is a really good idea though)

The two major problems with them:

  1. It's easy to inadvertently convert them back to normal strings. If a formatting API even calls "str" on the format string then we end up with a problem (and switching to containment instead of inheritance doesn't really help, since all objects implement str).

  2. They don't help with APIs that expect a percent-formatted string and do more with it than just pass it to str.mod (e.g. inspecting it for particular values such as '%(asctime)s')

Still, it's worth considering adding the three fmt_* classes to the string module to see how far they can get us in adapting the formats for different APIs.

Note that I don't think these concepts are fully baked yet, so we shouldn't do anything in a hurry - and anything that does happen should be via a PEP so we can flush out more issues.

Cheers, Nick.

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

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