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?

Related: stackoverflow.com/questions/42448664/…

dano
– dano

2019-08-06 13:59:16 +00:00
Commented Aug 6, 2019 at 13:59 — dano
– dano, Commented Aug 6, 2019 at 13:59

RomanPerekhrest · Accepted Answer · 2019-08-06 14:07:42Z

0

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

answered Aug 6, 2019 at 14:07

RomanPerekhrest

93.1k4 gold badges75 silver badges112 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Jwan622 Over a year ago

do I have to change anything else? Do I have to implemenet the __anext()) method?

RomanPerekhrest Over a year ago

@Jwan622, you may in case if you want to use a custom class implementation instead (python.org/dev/peps/pep-0525/#rationale-and-goals) But your case is too simple to bother

Jwan622 Over a year ago

wrote another quick question for you @RomanPerekhrest relatedly

Collectives™ on Stack Overflow

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

1 Answer 1

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related