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Mon Jan 13 23:46:01 CET 2014 · https://mail.python.org/pipermail/python-dev/2014-January/131469.html

On 1/13/2014 1:59 PM, Guido van Rossum wrote:
> On Mon, Jan 13, 2014 at 1:29 PM, Glenn Linderman <v+python at g.nevcal.com> wrote:
>> On 1/13/2014 12:09 PM, Guido van Rossum wrote:
>>
>> Yeah, the %s behavior with a string argument was a messy attempt at
>> compromise. I was hoping to mimick a common use of %s in Python 2,
>> where it can be used with either an 8-bit string or a number as
>> argument, acting like %b in the former case and like %d in the latter
>> case. Not having %s at all in Python 3 means that porting requires
>> more thinking (== more opportunity for mistakes when you're converting
>> in bulk) and there's no easy way to write code that works in Python 2
>> and 3.
>>
>> If we have %b for strictly interpolating bytes, I'm fine with adding
>> %a for calling ascii() on the argument and then interpolating the
>> result after ASCII-encoding it.
>>
>> If somehow (unlikely though it seems) we end up keeping %s (e.g.
>> strictly to ease porting), we could also keep %r as an alias for %a.
>>
>>
>> %s for strictly interpolating bytes eases porting. Sad name, but good for
>> compatibility. When the blowup happens, due to having a str type passed, the
>> porter adds the appropriate .encode(...) to the parameter, so it doesn't
>> blow up on Py 3, and it'll be OK for Py 2 as well, will it not?
> Lots of code uses %s with numbers too, and probably the occasional
> None or list (relying on the Python 2 near-guarantee that most
> objects' str() is their repr() and that repr() nearly guarantees to
> return only ASCII).
>
> E.g. I'm sure you can find live code doing something like
>
> headers.append('Content-Length: %s\r\n' % len(body))
>
That's portably fixable by switching to %d... or by adding .encode('ascii')
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[Python-Dev] PEP 460 reboot