await 何时会暂停 python 中可等待对象的执行？

Question

我已阅读，接受的答案提到：

It is worth mentioning that asyncio.sleep is explicitly guaranteed to suspend execution and defer to the event loop, even when the specified delay is 0 (in which case it will immediately resume on the next event loop pass)."

来自 python 文档 https://docs.python.org/3/library/asyncio-dev.html#concurrency-and-multithreading:

When a Task executes an await expression, the running Task gets suspended, and the event loop executes the next Task.

但是在我下面的例子中，在do_something中遇到await asyncio.sleep(0.1)的时候没有挂起，我的理解有什么问题吗？

import asyncio

async def do_something(i):
    await asyncio.sleep(0.1)
    print('in do_something')
    

async def main():
    for i in range(10):
        print(i)
        await do_something(i)


t1 = time.time()

loop = asyncio.get_event_loop()
loop.run_until_complete(main())
print('cost time:', time.time() - t1)

预期输出：

0
1
2
3
4
5
6
7
8
9
in do_something
in do_something
in do_something
in do_something
in do_something
in do_something
in do_something
in do_something
in do_something
in do_something
cost time: 1.0283539295196533

实际输出：

in do_something
1
in do_something
2
in do_something
3
in do_something
4
in do_something
5
in do_something
6
in do_something
7
in do_something
8
in do_something
9
in do_something
cost time: 1.0283539295196533

Answer 1

But in my following example, it didn't suspend when it meet await asyncio.sleep(0.1) in do_something

它确实暂停了，从某种意义上说，控制权在睡眠期间传递给了事件循环，允许它执行其他任务——如果有的话。要看到这一点，将 loop.run_until_complete(main()) 更改为 loop.run_until_complete(asyncio.gather(main(), main()))，您将观察到两个主电源并行执行。

正如文档所说，await 的意思是“调用这个异步函数，并暂停我直到它完成”。所以“await”意味着“等待”，如果你只有一个任务，等待会让一切看起来都是同步的。您可以将 await do_something(i) 更改为 asyncio.create_task(do_something(i))，并在 main 的末尾添加一个 await asyncio.sleep(10) （以防止程序结束），您应该会看到预期的输出。

await 何时会暂停 python 中可等待对象的执行？

When await will suspend the execution of an awaitable object in python?

python

async-await

python-asyncio