如何在异步函数中使用“ yield”？

Question

我想使用生成器yield和async函数。我阅读了本主题，并编写了以下代码：

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

但是我得到了错误：

语法错误：异步函数中的“ yield”

如何在异步功能中使用良率生成器？

Answer 1

更新：

从Python
3.6开始，我们有了异步生成器，能够yield直接在协程内部使用。

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：

您不能yield在协程内部。唯一的方法是使用/
magic方法手动实现异步迭代器。在您的情况下：__aiter__``__anext__

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())

输出：

0
1
4