Is async Python really asynchronous?

Question

Using Python 3.6 and asyncio and aiohttp I wrote a simple async program:

from aiohttp import ClientSession
import asyncio, ssl, time

base_url = 'https://my-base-url.com/api'

async def fetch(session, id):
    query_params = {'qp1':'v1','qp2':'v2', 'id': id}
    async with session.get(base_url, params=query_params, ssl=ssl.SSLContext()) as response:
        res_json = await response.json()
        if response.status == 200:
            time.sleep(2)
            min_rating = res_json.get('minRating')
            max_rating = res_json.get('maxRating')
            print("id = %s, min = %s, max = %s" % (id, min_rating, max_rating))


async def run(ids):
    tasks = []
    async with ClientSession() as session:
        for id in ids:
            task = asyncio.ensure_future(fetch(session, id))
            tasks.append(task)

        responses = await asyncio.gather(*tasks)
        return responses


if __name__ == '__main__':
    ids = [123, 456, 789]
    future = asyncio.ensure_future(run(ids))
    event_loop = asyncio.get_event_loop()
    event_loop.run_until_complete(future)

    print("\n\ndone")

The time.sleep(2) inside fetch(session, id) makes it seem like this program is not asynchronous because it gets one response, sleeps, gets another, sleeps, so on and so forth. When I remove the sleep call, it does seem to be async/concurrent because the responses come back in a random order. What is sleep doing in this case? Is it locking all threads? Why does it appear to be sequential instead of parallel?

time.sleep blocks everything; you want to hand back control to the event loop until some time in the future, so use asyncio.sleep. See e.g. stackoverflow.com/a/42282058/3001761. — jonrsharpe
– jonrsharpe, Commented Jun 28, 2018 at 16:16
async is cooperative multitasking. When you use time.sleep, you're not cooperating. — user2357112
– user2357112, Commented Jun 28, 2018 at 16:21

Netwave · Accepted Answer · 2018-06-28 16:17:53Z

5

time.sleep(2) is a synchronous (blocking) call hence you are stopping the asynchronous call with it, you should use await asyncio.sleep(2) which it will "liberate" the resource.

answered Jun 28, 2018 at 16:17

Netwave

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2 Comments

Chadderall Over a year ago

thanks for the quick response! I thought that was the case

Netwave Over a year ago

@Chadderall, please consider to mark the answer as accepted if it was helpful. stackoverflow.com/help/someone-answers

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