[Python-Dev] PEP 460 reboot

[Guido van Rossum]

On Sun, Jan 12, 2014 at 4:28 PM, Ethan Furman <ethan at stoneleaf.us> wrote:
> On 01/12/2014 03:55 PM, Guido van Rossum wrote:
>>
>> There's a lot of discussion about PEP 460 and I haven't read it all.
>> Maybe you all have already reached the same conclusion that I have.
>
>
> No, no agreement has been reached.  Your contribution is timely.
>
>
>
>> PEP 460 itself currently rejects support for %d, AFAIK on the basis
>> that bytes aren't necessarily ASCII. I think that's a misunderstanding
>> of the intention of the bytes type.
>
>
>> [...] this does not mean the bytes type isn't allowed to have a
>>
>> noticeable bias in favor of encodings that are ASCII supersets, even
>> if not all bytes objects contain such data [...]
>
>
>> IMO it's totally fine and consistent if b'%d' % 42 returns b'42' and
>> also for b'{}'.format(42) to return b'42' [...]
>>
>>
>> - byte literals: b'abc' (it's a syntax error to have a non-ASCII character
>> here)
>> - the upper() and lower() methods modify the ASCII letter positions
>> - int(b'42') == 42, float(b'3.14') == 3.14
>
>
> So if we allow the numeric modifiers [1], the only remaining question is do
> we allow %c and %s, and if so how do they behave?
>
> Guido?

Yes, all the numeric formatting codes such as %x, %o, %e, %f, %g
should all work, as should the padding, justification and and related
modifiers. E.g. b'%4x' %10 should return b'   a'.

%c looks simple enough too: With an int it should insert one byte,
insisting that the value is in range(256). With a bytes argument the
length should be 1. (I note that I can't remember ever using %c --
it's just there because it's in C.)

%s seems the trickiest: I think with a bytes argument it should just
insert those bytes (and the padding modifiers should work too), and
for other types it should probably work like %a, so that it works as
expected for numeric values, and with a string argument it will return
the ascii()-variant of its repr(). Examples:

b'%s' % 42 == b'42'
b'%s' % 'x' == b"'x'" (i.e. the three-byte string containing an 'x'
enclosed in single quotes)

I have to admin I didn't know about ascii(). It's nifty. :-)

> --
> ~Ethan~
>
>
> [1] modifiers is not the right word for %i, %x, etc, is it?  What is the
> correct term?

I'd interpret "modifiers" as the stuff that can go between the % and
the format letter, e.g. %04d or %-.3s. The term I'd use for %i, %x etc
would be numeric formatting codes.

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