Asyncio example in Python

Question

I am trying to learn native coroutine. I run the above example and I don't understand it.

Here is the example.

import asyncio

async def say_after(delay, what):
    await asyncio.sleep(delay)
    print(what)

async def main():
    task1 = asyncio.create_task(
        say_after(3, "A")
    )
    
    task2 = asyncio.create_task(
        say_after(2, "B")
    )

    print("----0")
    await task1
    print("----1")
    await task2

asyncio.run(main())

I heard that await waits for the task to finish. I expected to be printed in the order

A
B
----1

However, in the example above, ----1 is output after both B and A are printed.

The following is the output printed.

----0
B
A
----1

Why is this output created?

Do you know a site that has a collection of useful examples for studying native coroutine?

mkrieger1 · Accepted Answer · 2021-04-06 23:48:52Z

2

Here is what happens on a timeline:

After 0 seconds:

task1 starts
task2 starts
output ---0
await task1 blocks

After 2 seconds:

task2 prints B and finishes

After 3 seconds:

task1 prints A and finishes
await task1 returns
output ---1
await task2 returns (task was already finished 1 second ago)

answered Apr 6, 2021 at 23:48

mkrieger1

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1 Comment

Liu Yu Over a year ago

Why ------1 comes at the end? I thought the output should be ----0 ------1 B A. What part am I missing here?

Woodford · Accepted Answer · 2021-04-06 23:58:30Z

0

There's no guarantee in what order the async tasks are run. The only guarantee you have is when you await a task execution in main will stop until that task is complete. The following are all valid outputs of your program and you can't assume that you will get any one in particular:

A
B
----0
----1

B
A
----0
----1

----0
A
B
----1

----0
B
A
----1

A
----0
B
----1

B
----0
A
----1

A
----0
----1
B

----0
A
----1
B

answered Apr 6, 2021 at 23:58

Woodford

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5 Comments

mkrieger1 Over a year ago

---0 will always come before A and B because the two tasks sleep for 2 or 3 seconds, respectively, while the main function runs. A will always come after B because task2 wakes up 1 second earlier than task1.

mkrieger1 Over a year ago

tbh I don't know if it is guaranteed that sleep(2) wakes up earlier than sleep(3) but I think something would be seriously wrong if it didn't.

Woodford Over a year ago

@mkrieger1 In practice, almost certainly so. But there's no guarantee. A system where print is implemented internally as async and could take up to 4 seconds to complete could produce these results (silly example).

mkrieger1 Over a year ago

That's a convincing argument.

Liu Yu Over a year ago

Why ------1 comes at the end? I thought the output should be ----0 ------1 B A. What part am I missing here?

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