Deprecation warnings for the future async and await keywords in Python 3.6

[marco-buttu]

BPO	26182
Nosy	@brettcannon, @giampaolo, @ned-deily, @1st1, @marco-buttu, @AnishShah
Files	async_await.patch: Patch and related test issue_26182.patch

^{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/1st1'
closed_at = <Date 2016-09-15.16:51:13.036>
created_at = <Date 2016-01-22.19:06:06.579>
labels = ['interpreter-core', 'type-feature', 'release-blocker', 'expert-asyncio']
title = 'Deprecation warnings for the future async and await keywords in Python 3.6'
updated_at = <Date 2017-11-08.19:58:50.497>
user = 'https://github.com/marco-buttu'

bugs.python.org fields:

activity = <Date 2017-11-08.19:58:50.497>
actor = 'serhiy.storchaka'
assignee = 'yselivanov'
closed = True
closed_date = <Date 2016-09-15.16:51:13.036>
closer = 'yselivanov'
components = ['Interpreter Core', 'asyncio']
creation = <Date 2016-01-22.19:06:06.579>
creator = 'marco.buttu'
dependencies = []
files = ['41908', '44669']
hgrepos = []
issue_num = 26182
keywords = ['patch']
message_count = 24.0
messages = ['258833', '259038', '259394', '259409', '259411', '259412', '259413', '259534', '259887', '259888', '259892', '260097', '260118', '260177', '260225', '276091', '276097', '276118', '276493', '276575', '276576', '276577', '276585', '280355']
nosy_count = 7.0
nosy_names = ['brett.cannon', 'giampaolo.rodola', 'ned.deily', 'python-dev', 'yselivanov', 'marco.buttu', 'anish.shah']
pr_nums = []
priority = 'release blocker'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue26182'
versions = ['Python 3.6']

[marco-buttu]

I saw that async and await will become keywords in Python 3.7 :

https://www.python.org/dev/peps/pep-0492/#deprecation-plans

I enabled the deprecation warnings in Python 3.5.1 and Python 3.6 dev, and I noticed that assigning to async or await does not issue any deprecation warning:

$ python -Wd -c "import sys; print(sys.version); async = 33"
3.5.1 (default, Jan 21 2016, 19:59:28)
[GCC 4.8.4]

$ python -Wd -c "import sys; print(sys.version); async = 33"
3.6.0a0 (default:4b434a4770a9, Jan 12 2016, 13:01:29)
[GCC 4.8.4]

[brettcannon]

If someone wants to try and fix this, I would look at how the warning for the 'with' statement was handled (it will either be in the compiler while generating bytecode or somewhere in the parser, but I'm fairly certain it's the compiler).

[marco-buttu]

The check for the 'with' statement was handled in the parser (parsetok.c), and in Python 2.5 the warnings were enabled by default.
I do not know how to check if the deprecation warning are enabled, otherwise I would have tried to fix the problem.

[brettcannon]

I'm not quite sure what you mean by "do not know how to check if the deprecation warning are enabled". Do you mean how do you make sure you don't emit a DeprecationWarning if someone hasn't turned warnings on? If that's the case then if you look at some example code and read https://docs.python.org/3/c-api/exceptions.html#issuing-warnings you will notice that when you call the warning-related functions, all you have to do is check if the return value is < 0, then return like there is an exception raised. Otherwise the warnings module handles whether something will be shown.

[AnishShah]

I would like to work on this, if it is okay with Marco?
I look at the history of parsetok.c file and I think I can solve this issue.

Also, I have a doubt - PEP-492 says that "async and await names will be softly deprecated in CPython 3.5 and 3.6".
What exactly does "softly deprecate" mean? is it just same as throwing a warning?

[brettcannon]

I don't know what "softly deprecate" means. Hopefully someone involved with the PEP can answer that question.

[1st1]

I don't know what "softly deprecate" means. Hopefully someone involved with the PEP can answer that question.

I actually thought about emitting DeprecationWarnings in 3.6 for async/await NAME tokens. I think it's a reasonable thing to do to prepare for 3.7, where they will become proper keywords.

[marco-buttu]

Thank you Brett, your explanation (msg259409) and the c-api doc are really straightforward. I think I can fix it in a few days, just the time to read the devguide and be confident about the workflow.

[1st1]

Assigning the issue to myself to make sure it won't be forgotten before it's too late. Anish or Marco, feel free to propose a patch.

[vstinner]

Can you please mention the python version in the title?