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

Brett Cannon bcannon at gmail.com
Thu Mar 27 19:47:59 CET 2014

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 Pybuffer [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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