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

Brett Cannon brett at python.org
Sat Oct 3 20:27:20 CEST 2009

On Sat, Oct 3, 2009 at 11:01, Steven D'Aprano <steve at pearwood.info> wrote:

On Sun, 4 Oct 2009 01:41:36 am Steven Bethard wrote:

I thought it might be useful for those who don't have time to read a million posts to have a summary of what's happened in the formatting discussion.

The basic problem is that many APIs in the standard library and elsewhere support only %-formatting and not {}-formatting, e.g. logging.Formatter accepts:: logging.Formatter(fmt="%(asctime)s - %(name)s") but not:: logging.Formatter(fmt="{asctime} - {name}") Why is this a problem? Is it because the APIs require extra functionality that only {} formatting can provide? Possibly, but I doubt it -- I expect that the reason is: (1) Some people would like to deprecate % formatting, and they can't while the std lib uses % internally. (2) Some APIs in the std lib are tightly coupled to their internal implementation, and so you can't change their implementation without changing the API as well. Remove either of these issues, and the problem becomes a non-problem, and no action is needed. Personally, I'd like to see no further talk about deprecating % until at least Python 3.2, that is, the earliest we would need to solve this issue would be 3.3. As the Zen says, "Although never is often better than right now."

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.

-Brett

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