Deprecate old-style locking in asyncio/locks.py

[asvetlov]

BPO	32253
Nosy	@asvetlov, @serhiy-storchaka, @1st1
PRs	#4764

^{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 = None
closed_at = <Date 2017-12-09.18:00:28.630>
created_at = <Date 2017-12-08.08:59:40.579>
labels = ['3.7', 'library', 'expert-asyncio']
title = 'Deprecate old-style locking in asyncio/locks.py'
updated_at = <Date 2017-12-09.18:00:28.629>
user = 'https://github.com/asvetlov'

bugs.python.org fields:

activity = <Date 2017-12-09.18:00:28.629>
actor = 'asvetlov'
assignee = 'none'
closed = True
closed_date = <Date 2017-12-09.18:00:28.630>
closer = 'asvetlov'
components = ['Library (Lib)', 'asyncio']
creation = <Date 2017-12-08.08:59:40.579>
creator = 'asvetlov'
dependencies = []
files = []
hgrepos = []
issue_num = 32253
keywords = ['patch']
message_count = 5.0
messages = ['307839', '307887', '307888', '307903', '307905']
nosy_count = 3.0
nosy_names = ['asvetlov', 'serhiy.storchaka', 'yselivanov']
pr_nums = ['4764']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue32253'
versions = ['Python 3.7']

[asvetlov]

Now constructions like

await lock # "yield from lock" can be used as well
try:
...
finally:
lock.release()

and

with (yield from lock):
    ...

are supported. Let's deprecate them in favor of

async with lock:
    ...

[serhiy-storchaka]

This can make harder writing portable code that works in 2.7, 3.4 and 3.7.

What is the benefit of the deprecation? Are there inevitable design or implementation errors in these constructions? Or getting rid of them can significantly simplify the implementation?

[asvetlov]

1. asyncio is not supported by 2.7 anyway
2. with (yield from lock) is based on very non-obvious tricks:
   a) lock.enter is forbidden and raises RuntimeError
   b) actually lock.iter is called for lock acquiring before calling with
   c) the object returned from iter is a context manager with enter/exit methods
3. asyncio is converted to async/await syntax by https://bugs.python.org/issue32193 (old yield from style is fully supported still).
4. the deprecation was proposed by Yury Selivanov in #4753 (comment)

[1st1]

This can make harder writing portable code that works in 2.7, 3.4 and 3.7.

asyncio for Python 3.4 is fairly outdated. Most of the async packages today require 3.5+, as they usually use async/await syntax. I say this sort of backwards compatibility (showing a warning) isn't really a big concern. A bigger concern for us is new code using 'with await lock' pattern, hence the warning.

[asvetlov]

New changeset 28d8d14 by Andrew Svetlov in branch 'master':
bpo-32253: Deprecate with statement and bare await for asyncio locks (GH-4764)
28d8d14