[Python-Dev] PEP 461 - Adding % and {} formatting to bytes

Mark Lawrence breamoreboy at yahoo.co.uk
Fri Jan 17 16:15:58 CET 2014

On 17/01/2014 14:50, Eric V. Smith wrote:
> On 01/17/2014 07:34 AM, Eric V. Smith wrote:
>> On 1/17/2014 6:42 AM, Nick Coghlan wrote:
>>> On 17 Jan 2014 18:03, "Eric Snow" <ericsnowcurrently at gmail.com
>>> <mailto:ericsnowcurrently at gmail.com>> wrote:
>>>> On Thu, Jan 16, 2014 at 11:30 AM, Eric V. Smith <eric at trueblade.com
>>> <mailto:eric at trueblade.com>> wrote:
>>>>> For the first iteration of bytes.format(), I think we should just
>>>>> support the exact types of int, float, and bytes. It will call the
>>>>> type's__format__ (with the object as "self") and encode the result to
>>>>> ASCII. For the stated use case of 2.x compatibility, I suspect this will
>>>>> cover > 90% of the uses in real code. If we find there are cases where
>>>>> real code needs additional types supported, we can consider adding
>>>>> __format_ascii__ (or whatever name we cook up).
>>>> +1
>>> Please don't make me learn the limitations of a new mini language
>>> without a really good reason.
>>> For the sake of argument, assume we have a Python 3.5 with bytes.__mod__
>>> restored roughly as described in PEP 461. *Given* that feature set, what
>>> is the rationale for *adding* bytes.format? What new capabilities will
>>> it provide that aren't already covered by printf-style interpolation
>>> directly to bytes or text formatting followed by encoding the result?
>> The only reason to add any of this, in my mind, is to ease porting of
>> 2.x code. If my proposal covers most of the cases of b''.format() that
>> exist in 2.x code that wants to move to 3.5, then I think it's worth
>> doing. Is there any such code that's blocked from porting by the lack of
>> b''.format() that supports bytes, int, and float? I don't know. I
>> concede that it's unlikely.
>> IF this were a feature that we were going to add to 3.5 on its own
>> merits, I think we add __format_ascii__ and make the whole thing
>> extensible. Is there any new code that's blocked from being written by
>> missing b"".format()? I don't know that, either.
> Following up, I think this leaves us with 3 choices:
> 1. Do not implement bytes.format(). We tell any 2.x code that's written
> to use str.format() to switch to %-formatting for their common code base.
> 2. Add the simplistic version of bytes.format() that I describe above,
> restricted to accepting bytes, int, and float (and no subclasses). Some
> 2.x code will work, some will need to change to %-formatting.
> 3. Add bytes.format() and the __format_ascii__ protocol. We might want
> to also add a format_ascii() builtin, to match __format__ and format().
> This would require the least change to 2.x code that uses str.format()
> and wants to move to bytes.format(), but would require some work on the
> 3.x side.
> I'd advocate 1 or 2.
> Eric.

For both options 1 and 2 surely you cannot be suggesting that after 
people have written 2.x code to use format() as %f formatting is to be 
deprecated, they now have to change the code back to the way they may 
well have written it in the first place?

My fellow Pythonistas, ask not what our language can do for you, ask 
what you can do for our language.

Mark Lawrence

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