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On Mon, Jan 13, 2014 at 1:29 PM, Glenn Linderman  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))