[Python-Dev] PEP 461: Adding % formatting to bytes and bytearray -- Final, Take 3

[Brett Cannon]

On Thu Mar 27 2014 at 2:42:40 PM, Guido van Rossum <guido at python.org> wrote:

> Much better, but I'm still not happy with including %s at all. Otherwise
> it's accept-worthy. (How's that for pressure. :-)
>

But if we only add %b and leave out %s then how is this going to lead to
Python 2/3 compatible code since %b is not in Python 2? Or am I
misunderstanding you?

-Brett


>
>
> On Thu, Mar 27, 2014 at 11:04 AM, Ethan Furman <ethan at stoneleaf.us> wrote:
>
>> On 03/27/2014 10:55 AM, Ethan Furman wrote:
>>
>>> On 03/27/2014 10:29 AM, Guido van Rossum wrote:
>>>
>>>>
>>>> I also don't understand why we can't use %b instead of %s. AFAIK %b
>>>> currently doesn't mean anything and I somehow don't
>>>> expect we're likely to add it for other reasons (unless there's a
>>>> proposal I'm missing?). Just like we use %a instead of
>>>> %r to remind people that it's not quite the same (since it applies
>>>> .encode('ascii', 'backslashreplace')), shouldn't we
>>>> use anything *but* %s to remind people that that is also not the same
>>>> (not at all, in fact)? The PEP's argument against
>>>> %b ("rejected as not adding any value either in clarity or simplicity")
>>>> is hardly a good reason.
>>>>
>>>
>>> The biggest reason to use %s is to support a common code base for 2/3
>>> endeavors.  The biggest reason to not include %b
>>> is that it means binary number in format(); given that each type can
>>> invent it's own mini-language, this probably isn't
>>> a very strong argument against it.
>>>
>>> I have moderate feelings for keeping %s as a synonym for %b for
>>> backwards compatibility with Py2 code (when it's
>>> appropriate).
>>>
>>
>> Changed to:
>> ------------------------------------------------------------
>> ----------------------
>> ``%b`` will insert a series of bytes.  These bytes are collected in one
>> of two
>> ways:
>>
>>   - input type supports ``Py_buffer`` [4]_?
>>
>>     use it to collect the necessary bytes
>>
>>   - input type is something else?
>>     use its ``__bytes__`` method [5]_ ; if there isn't one, raise a
>> ``TypeError``
>>
>> In particular, ``%b`` will not accept numbers nor ``str``.  ``str`` is
>> rejected
>> as the string to bytes conversion requires an encoding, and we are
>> refusing to
>> guess; numbers are rejected because:
>>
>>   - what makes a number is fuzzy (float? Decimal? Fraction? some user
>> type?)
>>
>>   - allowing numbers would lead to ambiguity between numbers and textual
>>     representations of numbers (3.14 vs '3.14')
>>
>>   - given the nature of wire formats, explicit is definitely better than
>> implicit
>>
>> ``%s`` is included as a synonym for ``%b`` for the sole purpose of making
>> 2/3 code
>> bases easier to maintain.  Python 3 only code should use ``%b``.
>>
>> Examples::
>>
>>     >>> b'%b' % b'abc'
>>     b'abc'
>>
>>     >>> b'%b' % 'some string'.encode('utf8')
>>     b'some string'
>>
>>     >>> b'%b' % 3.14
>>
>>     Traceback (most recent call last):
>>     ...
>>     TypeError: b'%b' does not accept 'float'
>>
>>     >>> b'%b' % 'hello world!'
>>
>>     Traceback (most recent call last):
>>     ...
>>     TypeError: b'%b' does not accept 'str'
>> ------------------------------------------------------------
>> ----------------------
>>
>
>>
>> --
>> ~Ethan~
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>>
>
>
>
> --
> --Guido van Rossum (python.org/~guido)
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