async/await support doesn't work with Twisted

[dreid]

I'm not sure I expect cached-property to do anything about this, but it appears to be an undocumented limitation.

My (limited) understanding is that this is part cached_property limitation and part a limitation of Twisted's Deferred (it does not support being awaited from an asyncio coroutine which is what cached_property is trying to do).

Hopefully one of the active Twisted developers (or someone else) can chime in with how cached_property might handle this.

Versions:

• Python 3.8.2
• Twisted==20.3.0
• cached-property==1.5.1

Reproduction:

from cached_property import cached_property

class Monopoly(object):

    def __init__(self):
        self.boardwalk_price = 500

    @cached_property
    async def boardwalk(self):
        self.boardwalk_price += 50
        return self.boardwalk_price

async def print_boardwalk():
    monopoly = Monopoly()
    print(await monopoly.boardwalk)
    print(await monopoly.boardwalk)
    print(await monopoly.boardwalk)

print("With asyncio")
print("------------")
import asyncio
asyncio.get_event_loop().run_until_complete(print_boardwalk())
print("------------")

print("With twisted")
print("------------")
from twisted.internet.defer import ensureDeferred
from twisted.internet.task import react
react(lambda _: ensureDeferred(print_boardwalk()))
print("------------")

Results:

With asyncio
------------
550
550
550
------------
With twisted
------------
main function encountered error
Traceback (most recent call last):
  File ".ve/lib/python3.8/site-packages/twisted/internet/task.py", line 909, in react
    finished = main(_reactor, *argv)
  File "<stdin>", line 29, in <lambda>
    
  File ".ve/lib/python3.8/site-packages/twisted/internet/defer.py", line 911, in ensureDeferred
    return _cancellableInlineCallbacks(coro)
  File ".ve/lib/python3.8/site-packages/twisted/internet/defer.py", line 1529, in _cancellableInlineCallbacks
    _inlineCallbacks(None, g, status)
--- <exception caught here> ---
  File ".ve/lib/python3.8/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
    result = g.send(result)
  File "<stdin>", line 15, in print_boardwalk
    
  File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/asyncio/coroutines.py", line 127, in coro
    res = yield from res
builtins.RuntimeError: await wasn't used with future