Asyncio execution flow issue

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i am a little new to asyncio in python. I was trying to run this simple code but i don't know why i am getting this unexpected output.

What i did is that, in outer function, i created async tasks and stored it in an array tasks. Before awaiting on these tasks i wrote a print statement print("outer") that should run in every iteration. And inside the task i wrote another print statement print("inner") in inner function. But some how i am getting some unexpected output.

Here's the code - 

import asyncio


def main():
    loop = asyncio.get_event_loop()
    loop.run_until_complete(outer(loop))
    loop.close()


async def outer(loop):
    tasks = []
    for i in range(0, 5):
        tasks.append(loop.create_task(inner()))

    for task in tasks:
        print("outer")
        await task


async def inner():
    print("inner")
    await asyncio.sleep(0.5)

if __name__ == '__main__':
    main()

Here's the output -

outer
inner
inner
inner
inner
inner
outer
outer
outer
outer

My Expected output was -

outer
inner
outer
inner
outer
inner
outer
inner
outer
inner

Why all the inner are printing before outer. What is the correct execution flow of asyncio. Thanks in advance.

async-await python-asyncio asynchronous python
2 Answers

In async def outer(loop)

for i in range(0, 5):
    tasks.append(loop.create_task(inner()))
  • Five new inner tasks are created, scheduled and start running.
  • the event loop runs the scheduled tasks till they are completed.

If you add a little bit more to to inner and outer it shows this process better:

async def outer(loop):
    tasks = []
    for i in range(0, 5):
        tasks.append(loop.create_task(inner(i)))
    await asyncio.sleep(3)
    for task in tasks:
        print('outer')
        await task

async def inner(n):
    print(f"inner {n} start")
    await asyncio.sleep(0.5)
    print(f'inner {n} end')

  • while outer is sleeping

    • The first task runs to its await statement
    • The event loop suspends execution of the first task
    • The event loop runs the next schduled task up to its await statement
    • This continues till each task has had a chance to run up to its await statement then the event loop starts looking around to see if any of the tasks are done waiting for whatever they were waiting for - if done it lets them run some more.
    • This continues till all the tasks are done

  • You can see that the five tasks execute and finish before the second for loop even starts.

  • in the second for loop each task has already completed so await task has nothing to wait for and outer is printed five times in succession.

I'm a little hazy on the event loop controlling everything - I haven't found any explicit documentation for that - probably alluded to in the create_task docs: Wrap the coro coroutine into a Task and schedule its execution. When you create the task it gets scheduled. I have seen videos on pyvideo.org that shows that process, unfortunately I couldn't quickly find the one I wanted to link to.

user_28237550

The loop.create_task(inner()) immediately queues all 5 inner tasks for running, not await task. The await task suspends outer until the first task is completely done, i.e. at least 0.5 seconds. During this, all inner tasks have been run once up await, including their print.

async def outer(loop):
    tasks = []
    for i in range(0, 5):
        # queues task immediately
        tasks.append(loop.create_task(inner()))

    for task in tasks:
        print("outer")
        # suspends for at least 0.5 seconds
        await task


async def inner():
    # runs immediately
    print("inner")
    await asyncio.sleep(0.5)
user_53499160
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