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

James Y Knight foom at fuhm.net
Fri Oct 2 01:13:28 CEST 2009

On Oct 1, 2009, at 6:19 PM, Steven Bethard wrote:

I see how this could allow a user to supply a {}-format string to an API that accepts only %-format strings. But I still don't see the transition strategy for the API itself. That is, how does the %-format API use this to eventually switch to {}-format strings? Could someone please lay it out for me, step by step, showing what happens in each version?

Here's what I said in my first message, suggesting this change.
Copy&pasted below:

I wrote:

1) introduce the above feature, and recommend in docs that people only ever use new-style format strings, wrapping the string in newstyleformatstr() when necessary for passing to an API which uses % internally. 2) A long time later...deprecate str.mod; don't deprecate newstyleformatstr.mod. 3) A while after that (maybe), remove str.mod and replace all calls in Python to % (used as a formatting operator) with .format() so that the default is to use newstyle format strings for all APIs from then on.

So do (1) in 3.2. Then do (2) in 3.4, and (3) in 3.6. I skipped two
versions each time because of how widely this API is used, and the
likely pain that doing the transition quickly would cause. But I guess
you could do it in one version each step.

James

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