How to use 'yield' inside async function?

Question

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 1

Upd:

Starting with Python 3.6 we have asynchronous generators and able to use yield directly inside coroutines.

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()

Old answer for Python 3.5:

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