Python - What is the right way to run functions in asyncio?

Question

I am using FastApi and have one endpoint.

I have two long running functions which I want to run concurrently using asyncio

Therefore, I have created two functions:

async def get_data_one():
    return 'done_one'

async def get_data_two():
    return 'done_two'

These functions get data from external webservices.

I want to execute them concurrently so I have created a another function that does it:

async def get_data():
    loop = asyncio.get_event_loop()
    asyncio.set_event_loop(loop)
    task_1 = loop.create_task(get_data_one)
    task_2 = loop.create_task(get_data_two)
    tasks = (task_1, task_2)
    first, second = loop.run_until_complete(asyncio.gather(*tasks))
    loop.close()

    # I will then perform cpu intensive computation on the results
    # for now - assume i am just concatenating the results
    return first + second

Finally, I have my endpoint:

@app.post("/result")
async def get_result_async():
    r = await get_data()

    return r

Even this simple example breaks and I get the following exception when I hit the endpoint:

RuntimeError: This event loop is already running ERROR: _GatheringFuture exception was never retrieved future: <_GatheringFuture finished exception=AttributeError("'function' object has no attribute 'send'",)> AttributeError: 'function' object has no attribute 'send'

This is a simplified code but I would really appreciate how to do it the right way.

AKX · Accepted Answer · 2020-07-06 22:05:31Z

7

When in FastAPI context, you never need to run an asyncio loop; it's always running for as long as your server process lives.

Therefore, all you need is

import asyncio


async def get_data_one():
    return "done_one"


async def get_data_two():
    return "done_two"


async def get_data():
    a, b = await asyncio.gather(get_data_one(), get_data_two())
    return a + b


#@route decorator here...
async def get_result_async():
    r = await get_data()
    print("Get_data said:", r)
    return r


# (this is approximately what is done under the hood,
#  presented here to make this a self-contained example)
asyncio.run(get_result_async())

answered Jul 6, 2020 at 22:05

AKX

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2 Comments

InfoLearner Over a year ago

Do I need to lock any data in get_data function?or async will run each request in its own 'container' and I don't need to worry about making code aync/thread safe?

AKX Over a year ago

They're not run in isolation - if you have global state, you need to take care of it like you would with threads.

yuri kilochek · Accepted Answer · 2020-07-06 22:06:24Z

2

It's as simple as:

async def get_data():
    first, second = await asyncio.gather(
        get_data_one(),
        get_data_two(),
    )

    return first + second

answered Jul 6, 2020 at 22:06

yuri kilochek

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