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

"Martin v. Löwis" martin at v.loewis.de
Wed Sep 30 05:15:49 CEST 2009

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.

We should really provide some sort of transition plan.

-1.

Consider an example from the logging docs:

logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s") We'd like to support both this style as well as the following style: logging.Formatter()

Now, that's different. IIUC, you are not actually performing the substitution here, but only at a later place.

So changing to the new formatting mechanism is an API change. I agree that the new placeholder syntax needs to be supported.

* 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.

I would propose that the format argument gets an argument name, according to the syntax it is written in. For PEP 3101 format, I would call the argument "format" (like the method name of the string type), i.e.

logging.Formatter( format="{asctime} - {name} - {levelname} - {message}")

For the % formatting, I suggest "dicttemplate" (assuming that you have to use dictionary %(key)s style currently).

The positional parameter would also mean dicttemplate, and would be deprecated (eventually requiring a keyword-only parameter).

Regards, Martin

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