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