[functools] provide an async-compatible version of functools.lru_cache

[LiraNuna]

BPO	35040
Nosy	@brettcannon, @asvetlov, @1st1, @LiraNuna, @tirkarthi
Files	test-case.py

^{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 = None
created_at = <Date 2018-10-22.01:39:51.011>
labels = ['type-feature', '3.8', 'expert-asyncio']
title = '[functools] provide an async-compatible version of functools.lru_cache'
updated_at = <Date 2019-05-28.20:44:49.370>
user = 'https://github.com/LiraNuna'

bugs.python.org fields:

activity = <Date 2019-05-28.20:44:49.370>
actor = 'Liran Nuna'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['asyncio']
creation = <Date 2018-10-22.01:39:51.011>
creator = 'Liran Nuna'
dependencies = []
files = ['47887']
hgrepos = []
issue_num = 35040
keywords = []
message_count = 6.0
messages = ['328228', '328237', '328274', '343687', '343816', '343818']
nosy_count = 5.0
nosy_names = ['brett.cannon', 'asvetlov', 'yselivanov', 'Liran Nuna', 'xtreak']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue35040'
versions = ['Python 3.8']

[LiraNuna]

lru_cache is a very useful method but it does not work well with coroutines since they can only be executed once.

Take for example, the attached code (test-case.py) - It will throw a RuntimeError because you cannot reuse an already awaited coroutine.

A solution would be to call asyncio.ensure_future on the result of the coroutine if detected.

[asvetlov]

A coroutine detection is a relatively slow check.
I don't think we need to do it in functools.lru_cache.

There is a specialized asyncio compatible version: https://github.com/aio-libs/async_lru
Please use it.

[brettcannon]

Making this a feature request.

[asvetlov]

Brett please elaborate.
Do you want to incorporate async_lru library into CPython Core?

[brettcannon]

I was just saying that this is an enhancement request, no judgment about whether we want to solve the enhancement request.

[LiraNuna]

A coroutine detection is a relatively slow check.
I don't think we need to do it in functools.lru_cache.

Wouldn't a coroutine check only happen during decoration time? To successfully solve this easily and efficiently, we only really need to wrap the coroutine with asyncio.ensure_future if the decorated function is a coroutine, and it will only happen when a result comes back from the decorated function which would have minimal impact.

Of course, I don't know much about the internals of lru_cache so my assumptions could be wrong. I should familiar myself with the implementation and figure out how doable it would be.

[kumaraditya303]

Duplicate of #90780