Improve byte formatting compatibility between Py2 and Py3

[serhiy-storchaka]

BPO	23467
Nosy	@gvanrossum, @vstinner, @giampaolo, @ethanfurman, @serhiy-storchaka

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/ethanfurman'
closed_at = <Date 2015-03-11.15:17:31.841>
created_at = <Date 2015-02-15.18:33:27.344>
labels = ['interpreter-core', 'type-feature']
title = 'Improve byte formatting compatibility between Py2 and Py3'
updated_at = <Date 2015-03-14.19:49:14.197>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2015-03-14.19:49:14.197>
actor = 'ethan.furman'
assignee = 'ethan.furman'
closed = True
closed_date = <Date 2015-03-11.15:17:31.841>
closer = 'python-dev'
components = ['Interpreter Core']
creation = <Date 2015-02-15.18:33:27.344>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 23467
keywords = []
message_count = 8.0
messages = ['236057', '236062', '236063', '236068', '236069', '237874', '237885', '238099']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'vstinner', 'giampaolo.rodola', 'ethan.furman', 'python-dev', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue23467'
versions = ['Python 3.5']

[serhiy-storchaka]

The main motivation of PEP-461 was to help ease migration from, and/or have a single code base with, Python 2. But bytes formatting don't support the %r code supported in Python 2. Instead it supports the %a code which is exactly equivalent to the %r code in Python 2 but doesn't supported in Python 2. So it is not so easy to migrate from or have a single code base with Python 2 it the code uses the %r opcode.

As far as bytes formatting supports the %s code (an alias to %b) purely for compatibility with Python 2, it would be good to support the %r as an alias to %a.

[gvanrossum]

Yeah, we probably got carried away by purity concerns.

[vstinner]

"As far as bytes formatting supports the %s code (an alias to %b) purely for compatibility with Python 2, it would be good to support the %r as an alias to %a."

I don't like this idea.

[ethanfurman]

Sometimes practicality wins; it's why we allow %s, and we should also allow %r.

Both %s and %r need to be clearly documented as an aid to Py2/3 code bases, and not recommended for new code.

Serhiy, do you have time to take of this?

[serhiy-storchaka]

The implementation is easy -- just add "case 'r':" before "case 'a':". The hard (to me) part is the documentation, so that I'm out of the game.

[python-dev]

New changeset 611fa301b807 by Ethan Furman in branch 'default':
Close bpo-23467: add %r compatibility to bytes and bytearray
https://hg.python.org/cpython/rev/611fa301b807

[vstinner]

You should also update the PEP-461.

[ethanfurman]

Added in
https://hg.python.org/peps/rev/7fe79194a4f2