Why does `await asyncio.create_task()` behave different then when assigning it to a variable?

Question

Why is there a different how tasks are run between tasks 1-3 and 4-6 in the code below?

Code:

import asyncio


async def do_something(i, sleep):  # No I/O here
    print("Doing... ", end="")
    print(await no_io_1(i, sleep))


async def no_io_1(i, sleep):  # No I/O here
    return await no_io_2(i, sleep)


async def no_io_2(i, sleep):  # No I/O here
    return await io(i, sleep)


async def io(i, sleep):
    await asyncio.sleep(sleep)  # Finally some I/O
    # t = asyncio.create_task(asyncio.sleep(sleep))
    # await t
    return i


async def main():
    await asyncio.create_task(do_something(1, sleep=4))
    await asyncio.create_task(do_something(2, sleep=3))
    await asyncio.create_task(do_something(3, sleep=2))

    t4 = asyncio.create_task(do_something(4, sleep=4))
    t5 = asyncio.create_task(do_something(5, sleep=3))
    t6 = asyncio.create_task(do_something(6, sleep=2))
    await t4
    await t5
    await t6

asyncio.run(main())
print("\r\nBye!")

Output:

Doing... 1
Doing... 2
Doing... 3
Doing... Doing... Doing... 6
5
4

user4815162342 · Accepted Answer · 2022-02-04 08:39:02Z

In the first snippet you immediately await each task you've created. As a result, the tasks are prevented from running in parallel.

In the second snippet you create three tasks and only then start awaiting. This allows all three to run in parallel, despite your await specifying you're interested in the result of the first one. Being able to run other tasks while waiting for the results of a specific one is essential to libraries like asyncio.

In other words, await t1 doesn't mean "run t1", it means "suspend me and spin the event loop until t1 is done". The difference has nothing to do with tasks being stored in variables, but with creating tasks in advance. For example, you could modify the second example like this:

    t4 = asyncio.create_task(do_something(4, sleep=4))
    await t4
    t5 = asyncio.create_task(do_something(5, sleep=3))
    await t5
    t6 = asyncio.create_task(do_something(6, sleep=2))
    await t6

...and you would get behavior like that of the first example.

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