CODEWITHSUNDEEP
python-3.xpython-asyncio
ttyridal
ttyridal

Reputation: 468

Two way communication with coroutine (state machine in asyncio)

With send and yield we can have two-way communication with a generator and implement a state machine quite nicely (see example below). Now, we can't (?) send to asyncio coroutines, so how could one implement a state machine with asyncio-coroutines?

Generator example

def lock():
    combination = [1, 2, 3]
    for digit in combination:
        a = (yield True)
        while a != digit:
            a = (yield False)

    yield "You're in"


def main():
    l = lock()
    next(l)
    assert l.send(2) == False
    assert l.send(1) == True  # correct value 1st digit
    assert l.send(1) == False
    assert l.send(2) == True  # correct value 2nd digit
    assert l.send(2) == False
    assert l.send(3) == "You're in"  # correct value 3rd digit

Something similar with asyncio is not quite as nice.. Is there a better way?

asyncio proposal

import asyncio


class AsyncLock:
    class Message:
        def __init__(self, value):
            self.f = asyncio.Future()
            self.value = value

        def set_result(self, v):
            self.f.set_result(v)

        async def result(self):
            return await self.f

    def __init__(self, msg_q):
        self.msg_q = msg_q
        self.task = None

    async def send(self, value):
        msg = AsyncLock.Message(value)
        await self.msg_q.put(msg)
        return await msg.result()

    # all of the above to be able to do this:
    async def run(self):
        combination = [1, 2, 3]
        for digit in combination:
            msg = await self.msg_q.get()
            while msg.value != digit:
                msg.set_result(False)
                msg = await self.msg_q.get()
            msg.set_result("You're in" if digit == 3 else True)


async def amain():
    l = AsyncLock(asyncio.Queue())
    l.task = asyncio.ensure_future(l.run())

    assert await l.send(2) == False
    assert await l.send(1) == True
    assert await l.send(1) == False
    assert await l.send(2) == True
    assert await l.send(2) == False
    assert await l.send(3) == "You're in"

asyncio.get_event_loop().run_until_complete(amain())

Upvotes: 3

Views: 1695

Answers (1)

D.Shawley
D.Shawley

Reputation: 59553

Python3.6 added support for async generators (PEP525) so async functions can now be generators too!

#!/usr/bin/env python3.6

import asyncio

async def lock():
    combination = [1, 2, 3]
    for digit in combination:
        a = (yield True)
        while a != digit:
            a = (yield False)
    yield "You're in!"

async def main():
    coro = lock()
    await coro.asend(None)
    assert (await coro.asend(2)) == False
    assert (await coro.asend(1)) == True
    assert (await coro.asend(1)) == False
    assert (await coro.asend(2)) == True
    assert (await coro.asend(2)) == False
    assert (await coro.asend(3)) == "You're in!"
    print('Got it')

iol = asyncio.get_event_loop()
iol.run_until_complete(main())

Before Python3.6, the best approach is to use a message queue as you were doing.

Upvotes: 5

Related Questions