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

Guido van Rossum guido at python.org
Wed Sep 30 19:37:20 CEST 2009

On Wed, Sep 30, 2009 at 9:48 AM, "Martin v. Löwis" <martin at v.loewis.de> wrote:

I'd note that PEP 3101 calls str.format() a replacement for %-formatting, not an alternate mechanism to achieve the same end. I think this is a mis-wording; the intent of the PEP apparently is to propose this mechanism as an option, not as an actual replacement. This becomes clear when reading the "Backwards Compatibility" section:

The problem is, PEP 3101 and our interpretation of it evolved. The original proposal for {}-formatting was certainly put forward with the aim to completely replace %-formatting, and care was taken in the design to cover all use cases, avoid known problems, etc.

Then we started looking seriously at conversion from Python 2 to Python 3 and we discovered that converting %-formatting to {}-formatting was a huge can of worms, and decided it wasn't worth to try and do at the time given the Python 3 schedule. We considered some kind of gentle deprecation warning, but decided that even that would be too noisy. So now we have two competing mechanisms.

In the long run, say Python 4, I think we don't need both, and we should get rid of one. My preference is still getting rid of %-formatting, due to the problems with it that prompted the design of {}-formatting (no need to reiterate the list here).

So how do we get there? My proposal would be to let this be a gradual take-over of a new, superior species in the same niche as an older species. (Say, modern man over Neanderthal man.) Thus, as new code is written (especially example code, which will be copied widely), we should start using {}-formatting, and when new APIs are designed that tie in to some kind of formatting, they should use {}-formatting. Adding support for {}-formatting, in addition to %-formatting, to existing APIs like the logging package also strikes me as a good idea, as long as backwards compatibility can be preserved. (I have no strong ideas on how to do this right now.)

If we do this right, by the time Python 4 comes around, {}-formatting will have won the race, and there won't be a question about removing %-formatting at the time. I wouldn't be surprised if by then static analysis techniques will have improved so that we can consider automatic conversion by then.

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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