如何實作 async 的 generator

但只用純 Python 的 generator 沒辦法為 Python 的所有功能實作 async variant，因為這會有雞生蛋生雞問題。最簡單的矛盾：如果你只用 generator 實作 async API，那要怎麼實作 async 的 generator？

嗯？為什麼不行？

import asyncio
import time

def async_generator():
    @asyncio.coroutine
    def count_down(i, c):
        print('log from {}th async function with countdown {} and timestamp {}'.format(i, c, time.time()))
        if c == 0: return
        yield from asyncio.sleep(0.5)
        yield from count_down(i, c - 1)

    i = 1
    while i <= 5:
        yield count_down(i, i)
        i += 1

@asyncio.coroutine
def run_all(asyncGen):
    for a in asyncGen:
        yield from a

loop = asyncio.get_event_loop()
loop.run_until_complete(run_all(async_generator()))
loop.close()

output:

log from 1th async function with countdown 1 and timestamp 1494816543.0441651
log from 1th async function with countdown 0 and timestamp 1494816543.549279
log from 2th async function with countdown 2 and timestamp 1494816543.549315
log from 2th async function with countdown 1 and timestamp 1494816544.054571
log from 2th async function with countdown 0 and timestamp 1494816544.5569139
log from 3th async function with countdown 3 and timestamp 1494816544.5569532
log from 3th async function with countdown 2 and timestamp 1494816545.060334
log from 3th async function with countdown 1 and timestamp 1494816545.564309
log from 3th async function with countdown 0 and timestamp 1494816546.0687401
log from 4th async function with countdown 4 and timestamp 1494816546.0687969
log from 4th async function with countdown 3 and timestamp 1494816546.573986
log from 4th async function with countdown 2 and timestamp 1494816547.075812
log from 4th async function with countdown 1 and timestamp 1494816547.581194
log from 4th async function with countdown 0 and timestamp 1494816548.086391
log from 5th async function with countdown 5 and timestamp 1494816548.086432
log from 5th async function with countdown 4 and timestamp 1494816548.5881221
log from 5th async function with countdown 3 and timestamp 1494816549.08834
log from 5th async function with countdown 2 and timestamp 1494816549.5930529
log from 5th async function with countdown 1 and timestamp 1494816550.0984662
log from 5th async function with countdown 0 and timestamp 1494816550.603544

refs:

https://gist.github.com/rayshih/e7d736e19d172d1e1ebb127ffc26fe2b

rayshih/async-generator.md

rayshih commented May 15, 2017