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

Steven Bethard steven.bethard at gmail.com
Fri Oct 2 03:13:12 CEST 2009

On Thu, Oct 1, 2009 at 4:35 PM, Brett Cannon <brett at python.org> wrote:

On Thu, Oct 1, 2009 at 15:19, Steven Bethard <steven.bethard at gmail.com> wrote:

On Thu, Oct 1, 2009 at 11:03 AM, Brett Cannon <brett at python.org> wrote:

class bracesfmt(str):

 def mod(self, stuff):  if isinstance(stuff, tuple):  return self.class(self.format(*stuff))  elif isinstance(stuff, collections.Mapping):  return self.class(self.format(**stuff))  else:  return self.class(self.format(stuff)) The biggest issue is that "%s" % {'a': 42} substitutes the dict instead of throwing an error that str.format() would do with the code above. But what's nice about this is I think I can use this now w/ any library that expects % interpolation and it should basically work. Could someone please lay it out for me, step by step, showing what happens in each version? 1. Nothing changes as hopefully the wrapper works fine (as people are pointing out, though, my approach needs to override str() to return 'self', else the str type will just return what it has internally in its buffer). 2. Raise a deprecation warning when isinstance(ob, bracefmt) is false. When a class is passed in that is a subclass of bracefmt, call ob.format() on it. 3. Require the subclass. 4. Remove the requirement and always call ob.format().

Thanks Brett, that's clear. So you save one version over the proposal of adding a format= flag to the API.

On Thu, Oct 1, 2009 at 4:13 PM, James Y Knight <foom at fuhm.net> wrote:

Here's what I said in my first message, suggesting this change. Copy&pasted below: [snip steps that only talk about str.mod, not an API that uses it]

I didn't understand how you wanted to apply your suggestion to an API (instead of str.mod) the first time and I still don't understand it. Is what Brett has proposed the same thing?

Steve

Where did you get that preposterous hypothesis? Did Steve tell you that? --- The Hiphopopotamus

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