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

Paul Moore p.f.moore at gmail.com
Thu Oct 1 15:11:27 CEST 2009

2009/10/1 Vinay Sajip <vinay_sajip at yahoo.co.uk>:

If any module wants to use {} formatting for their logging, they can add the line

from logging import BraceMessage as __ I've used two underscores, since might be being used for gettext, but obviously the importer can use whatever name they want. and then they can use logger.debug(("The {0} is {1}", "answer", 42)) which I think is more readable than putting in ".format" following the string literal. It's not a huge point, perhaps, but "Readability counts". This has the side benefit that if e.g. Barry wanted to use string.Template for formatting, he's just got to replace the above import with something like _from logging import DollarMessage as __ Another "working title", please note. And while I've shown these classes being imported from logging, it doesn't make sense to put them there if this idea were to fly in a more general context. Then, perhaps string would be a better home for these classes.

This seems to me to be almost the same as the previous suggestion of having a string subclass:

class BraceFormatter(str): def mod(self, other): # Needs more magic here to cope with dict argument return self.format(*other)

__ = BraceFormatter

logger.debug(__("The {0} is {1}"), "answer", 42)

The only real differences are

  1. The positioning of the closing parenthesis
  2. The internal implementation of logger.debug needs to preserve string subclasses properly

But the benefit is that the approach allows anyone to use brace formatting in any API that currently accepts % format (assuming string subclasses don't get mangled).

On the one hand, I'd prefer a more general solution. On the other, I'm nervous about that "assuming string subclasses..." proviso.

I've no real answer, just offering the point up for consideration.

Paul.

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