[Python-Dev] cpython (3.2): Issue #14123: Explicitly mention that old style % string formatting has caveats (original) (raw)

Guido van Rossum guido at python.org
Mon Feb 27 00:33:51 CET 2012

On Sun, Feb 26, 2012 at 3:14 PM, Terry Reedy <tjreedy at udel.edu> wrote:

On 2/26/2012 5:38 PM, Nick Coghlan wrote:

Ah, thanks, I knew there was another term that had a new-style counterpart: percent formatting vs brace formatting. Hooray! Exact parallel and value-neutral.

Can we stop it with the "political correctness" already? The old style is best named printf-style formatting because that's the origin of the format language, and there are many other programming languages that support the same formatting language (with minor variations). I care less about what we call the new style -- "new style" or "format method" both work for me.

I also would like to suggest that, even if the reality is that we can't deprecate it today, eventually, at some distant point in the future we ought to start deprecating printf-style formatting -- it really does have a couple of nasty traps that keep catching people unawares. In the mean time it doesn't hurt to use terms that make people ever so slightly uneasy with using the old style for new code, while also committing to not throwing it out until Python 4 comes around.

That said, for consistency's sake, if you add formatting code to an existing module that uses the old style, please stick to the old style. And to avoid disasters, also please don't go on a library-wide rampage of wholesale conversions. The time to start using the new formatting is when writing new modules or packages, or possibly when doing a major refactoring/upgrade of an existing module or package.

One thing I'd like to see happening regardless is support for new-style formatting in the logging module. It's a little tricky to think how that would work, alas -- should this be a property of the logger or of the call?

-- --Guido van Rossum (python.org/~guido)

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