如何在异步函数中使用“yield”？

我想使用生成器产量和异步函数。我读这个话题 https://stackoverflow.com/questions/231767/what-does-the-yield-keyword-do-in-python，并编写了下一个代码：

import asyncio

async def createGenerator():
    mylist = range(3)
    for i in mylist:
        await asyncio.sleep(1)
        yield i*i

async def start():
    mygenerator = await createGenerator()
    for i in mygenerator:
        print(i)

loop = asyncio.get_event_loop()

try:
    loop.run_until_complete(start())

except KeyboardInterrupt:
    loop.stop()
    pass

但我收到错误：

语法错误：异步函数内的“yield”

如何在异步函数中使用收益生成器？

Upd:

从 Python 3.6 开始，我们有异步发电机 https://www.python.org/dev/peps/pep-0525/并能够使用yield直接在协程内部。

import asyncio


async def async_generator():
    for i in range(3):
        await asyncio.sleep(1)
        yield i*i


async def main():
    async for i in async_generator():
        print(i)


loop = asyncio.get_event_loop()
try:
    loop.run_until_complete(main())
finally:
    loop.run_until_complete(loop.shutdown_asyncgens())  # see: https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.loop.shutdown_asyncgens
    loop.close()

Python 3.5 的旧答案：

你不能yield在协程内部。唯一的办法就是落实异步迭代器 https://www.python.org/dev/peps/pep-0492/#asynchronous-iterators-and-async-for手动使用__aiter__/__anext__魔法方法。在你的情况下：

import asyncio


class async_generator:
    def __init__(self, stop):
        self.i = 0
        self.stop = stop

    async def __aiter__(self):
        return self

    async def __anext__(self):
        i = self.i
        self.i += 1
        if self.i <= self.stop:
            await asyncio.sleep(1)
            return i * i
        else:
            raise StopAsyncIteration


async def main():
    async for i in async_generator(3):
        print(i)


if __name__ == "__main__":
    loop = asyncio.get_event_loop()
    loop.run_until_complete(main())

Output:

0
1
4

这里还有两个例子：1 https://stackoverflow.com/a/36094436/1113207, 2 https://stackoverflow.com/a/37286577/1113207