Howdy,
I'm experimenting with async/await in Python 3, and one very surprising
behavior has been what happens when calling `await` twice on an Awaitable.
In C#, Hack/HHVM, and the new async/await spec in Ecmascript 7. In Python,
calling `await` multiple times results in all future results getting back
`None`. Here's a small example program:
async def echo_hi():
result = ''
echo_proc = await asyncio.create_subprocess_exec(
'echo', 'hello', 'world',
stdout=asyncio.subprocess.PIPE,
stderr=asyncio.subprocess.DEVNULL)
result = await echo_proc.stdout.read()
await echo_proc.wait()
return result
async def await_twice(awaitable):
print('first time is {}'.format(await awaitable))
print('second time is {}'.format(await awaitable))
loop = asyncio.get_event_loop()
loop.run_until_complete(await_twice(echo_hi()))
This makes writing composable APIs using async/await in Python very
difficult since anything that takes an `awaitable` has to know that it
wasn't already awaited. Also, since the behavior is radically different
than in the other programming languages implementing async/await it makes
adopting Python's flavor of async/await difficult for folks coming from a
language where it's already implemented.
In C#/Hack/JS calls to `await` return a Task/AwaitableHandle/Promise that
can be awaited multiple times and either returns the result or throws any
thrown exceptions. It doesn't appear that the Awaitable class in Python
has a `result` or `exception` field but `asyncio.Future` does.
Would it make sense to shift from having `await` functions return a `
*Future-like`* return object to returning a Future?
Thanks,
Roy
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