[Python-Dev] PEP 460 reboot

Guido van Rossum guido at python.org
Tue Jan 14 21:54:29 CET 2014

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'
> % data_value[:15] everywhere.

Wow. I thought there would be some combination using %.15s but I can't
get that to work. :-(

> 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.)

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

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