[Python-Dev] PEP 460 reboot (original) (raw)

MRAB python at mrabarnett.plus.com
Tue Jan 14 22:34:35 CET 2014

On 2014-01-14 20:54, Guido van Rossum wrote:

On Tue, Jan 14, 2014 at 12:13 PM, Ethan Furman <ethan at stoneleaf.us> wrote:

On 01/14/2014 10:52 AM, Guido van Rossum wrote:

Which reminds me. Quite a few people have spoken out in favor of loud failures rather than silent "wrong" output. But I think that in the specific context of formatting output, there is a long and IMO good tradition of producing (slightly) wrong output in favor of more strict behavior. Consider for example what to do when a number doesn't fit in the given width. Would you rather raise an exception, truncate the value, or mess up the formatting? One more data point to consider: When the binary format has strict rules on how much space a data-point is allowed, then failure is the only appropriate option. Yes, that's how the struct module works. In Py2, because '%15s' can actually take 17 characters, I have to use '%15s' % datavalue[:15] everywhere. Wow. I thought there would be some combination using %.15s but I can't get that to work. :-( I've not sure what you mean here:

Python 2.7.5 (default, May 15 2013, 22:44:16) [MSC v.1500 64 bit (AMD64)] on win 32 Type "help", "copyright", "credits" or "license" for more information.

import string '%.15s' % string.letters 'abcdefghijklmno' len(_) 15

I'm not suggesting we change how that portion works, as it would then be, I think, too different from both Py2 behavior as well as current str behavior, but likewise adding in single quotes would of no help to me. Loud failure so I can easily see where I forgot the .encode() would be much more helpful. If we go with a more restricted version this makes sense indeed. The single quotes seemed unavoidable when I was trying (like several other proposals) to have a format code that works for all types. I think we're rightly giving up on that now. (I should review PEP 461, but I don't have time yet.)

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