Ильдар Ильдар - 6 months ago 84
Python Question

How to use 'yield' inside async function?

I want to use generator yield and async functions. I read this topic, and wrote next code:

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


But i got the error:

SyntaxError: 'yield' inside async function

How to use yield generator in async function?

Answer

You can't yield inside coroutines. Only way is to implement Asynchronous Iterator manually using __aiter__/__anext__ magic methods. In your case:

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

Here're two more examples: 1, 2