How to fix this async generator object is not an iterator issue in Python?

Question

I'm trying to mock a websockets data stream and I'm getting this error: 'async_generator' object is not an iterator

This is my generator code:

from time import sleep


mock_sf_record = '{"payload": ...}'


async def generateMessages():
    sleep(5)
    yield mock_sf_record

and the code that calls this code:

async def subscribe(subscription):
    global RECEIVED_MESSAGES_CACHE
    ...
    while True:
        messageStream = await(next(generateMessages())) if ENV == 'dev' else await websocket.recv()

What can I do? What am I doing wrong? I'm basically using the generateMessages() generator to create a stream of messages, but this isn't working...

The code that is calling subscribe :

 for subscription in SUBSCRIPTION_TYPES:
        loop.create_task(subscribe(subscription))
    loop.run_forever()

More importantly, if I change the code to use a synchronous generator, this only generates messages for a single subscription and I never seem to generate messsages for any other subscription... it seems to block on a single thread. Why is this?

messageStream = (next(generateMessages())) if ENV == 'dev' else await websocket.recv()

and

# generator that generates mock SF data
from asyncio import sleep

mock_sf_record = '{"payload": ...}'


def generateMessages():
    sleep(5)
    yield mock_sf_record

Why does the synchronous generator cause problems?

Answer 1

The right way:

async def subscribe(subscription):
    global RECEIVED_MESSAGES_CACHE
    ...
    gen = generateMessages()    # init async generator
    messageStream = (await gen.__anext__()) if ENV == 'dev' else (await websocket.recv())

https://www.python.org/dev/peps/pep-0525/#support-for-asynchronous-iteration-protocol