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

Eric Smith eric at trueblade.com
Wed Sep 30 13:15:45 CEST 2009

Martin v. Löwis wrote:

Steven Bethard wrote:

There's a lot of code already out there (in the standard library and other places) that uses %-style formatting, when in Python 3.0 we should be encouraging {}-style formatting. I don't agree that we should do that. I see nothing wrong with using % substitution.

It's a maintenance burden. There are several outstanding bugs with it, admittedly not of any great significance. I've been putting time into fixing at least one of them. When Mark and I did short-float-repr, at least half of my time was consumed with %-formatting, mostly because of how it does memory management.

On the plus side, %-formatting is (and always will be) faster than str.format(). Its very limitations make it possible for it to be fast.

I'd note that PEP 3101 calls str.format() a replacement for %-formatting, not an alternate mechanism to achieve the same end.

* Add a new class, NewFormatter, which uses the {}-style formatting. This is ugly because it makes the formatting we're trying to encourage look like the alternative implementation instead of the standard one. It's also ugly because the class has the word "new" in its name, which no class should ever have. In a few years, it would still be around, but not new anymore.

* Have Formatter convert all %-style formatting strings to {}-style formatting strings (automatically). This still involves some guessing, and involves some serious hacking to translate from one to the other (maybe it wouldn't even always be possible?). But at least we'd only be using {}-style formatting under the hood. I don't see the point of having a converter. The tricky part, as you say, is the guessing. Whether the implementation then converts the string or has two alternative formatting algorithms is an implementation detail. I would favor continued use of the actual % substitution code.

Having a converter and guessing are 2 distinct issues. I believe a convert from %-formatting specification strings to str.format() strings is possible. I point this out not because I think a converter solves this problem, but because in the past there's been a debate on whether a converter is even possible.

Eric.

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