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

Daniel Holth dholth at gmail.com
Thu Mar 27 19:52:41 CET 2014

I feel not including %s is nuts. Should I write .replace('%b', '%s')? All I desperately need are APIs that provide enough unicode / str type safety that I get an exception when mixing them accidentally... in my own code, dynamic typing is usually a bug. As has been endlessly discussed, %s for bytes is a bit like exposing sprintf()...

On Thu, Mar 27, 2014 at 2:41 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. :-)

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

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