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

[Nick Coghlan]

On 15 Jan 2014 08:23, "Ethan Furman" <ethan at stoneleaf.us> wrote:
>
> On 01/14/2014 02:17 PM, Nick Coghlan wrote:
>>
>>
>> On 15 Jan 2014 07:36, "Ethan Furman" <ethan at stoneleaf.us <mailto:
ethan at stoneleaf.us>> wrote:
>>>
>>>
>>> On 01/14/2014 12:57 PM, Antoine Pitrou wrote:
>>>>
>>>>
>>>> On Tue, 14 Jan 2014 11:56:25 -0800
>>>> Ethan Furman <ethan at stoneleaf.us <mailto:ethan at stoneleaf.us>> wrote:
>>>>>
>>>>>
>>>>>
>>>>> %s, because it is the most general, has the most convoluted
resolution:
>>>>>
>>>>>     - input type is bytes?
>>>>>       pass it straight through
>>>>
>>>>
>>>>
>>>> It should try to get a Py_buffer instead.
>>>
>>>
>>>
>>> Meaning any bytes or bytes-subtype will support the Py_buffer protocol,
and this should be the first thing we try?
>>>
>>> Sounds good.
>>>
>>> For that matter, should the first test be "does this object support
Py_buffer" and not worry about it being isinstance(obj, bytes)?
>>
>>
>> Yep. I actually suggest adjusting the %s handling to:
>>
>> - interpolate Py_buffer exporters directly
>> - interpolate __bytes__ if defined
>> - reject anything with an "encode" method
>> - otherwise interpolate str(obj).encode("ascii")
>>
>>>>>     - input type is numeric?
>>>>>       use its __xxx__ [1] [2] method and ascii-encode it (strictly)
>>>>
>>>>
>>>>
>>>> What is the definition of "numeric"?
>>>
>>>
>>>
>>> That is a key question.
>>
>>
>> As suggested above, I would flip the question and explicitly *disallow*
implicit encoding of any object with its own
>> "encode" method, while allowing everything else.
>
>
> Um, int and floats (for example) don't have an .encode method, don't
export Py_buffer, don't have a __bytes__ method... Ah! so it would hit the
last case, I see.
>
> The danger I see with that route is that any ol' object could then make
it into the byte stream, and considering what byte streams are for I think
we should make the barrier for entry higher than just relying on a __str__
or __repr__.

Yeah, reading the other thread pointed out the issues with this idea
(containers in particular are a problem).

I think Brett has the right idea: we shouldn't try to accept numbers for %s
in binary interpolation. If we limit it to just buffer exporters and
objects with a __bytes__ method then the problem goes away.

The numeric codes all exist in Python 2, so the porting requirement to the
common 2/3 subset will be to update the cases of binary interpolation of a
number with %s to use an appropriate numeric formatting code instead.

Cheers,
Nick.

>
>
> --
> ~Ethan~
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