python - asyncio.create_task 与等待

Question

我无法理解 asyncio.create_task() 是如何工作的Python 3.7 中引入的函数应该可以工作。如果我这样做:

import asyncio

async def helloworld():
    print("Hello world from a coroutine!")
    asyncio.create_task(helloworld())

def main():
    loop = asyncio.get_event_loop()
    loop.run_until_complete(helloworld())

if __name__ == "__main__":
    main()

我得到:

Hello world from a coroutine!
Hello world from a coroutine!

作为输出(即协程运行两次)。这怎么不是无限递归呢？当我使用 await 时，我希望看到我所看到的关键词:

import asyncio


async def helloworld():
    print("Hello world from a coroutine!")
    await helloworld()


def main():
    loop = asyncio.get_event_loop()
    loop.run_until_complete(helloworld())


if __name__ == "__main__":
    main()

有了这个我得到:


Hello world from a coroutine!
Hello world from a coroutine!
Hello world from a coroutine!
... many more lines...
Traceback (most recent call last):
File "test3.py", line 53, in <module>
main()
File "test3.py", line 48, in main
loop.run_until_complete(helloworld())
File "/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/base_events.py", line 568, in run_until_complete
return future.result()
File "test3.py", line 37, in helloworld
await helloworld()
File "test3.py", line 37, in helloworld
await helloworld()
File "test3.py", line 37, in helloworld
await helloworld()
[Previous line repeated 984 more times]
File "test3.py", line 36, in helloworld
print("Hello world from a coroutine!")
RecursionError: maximum recursion depth exceeded while calling a Python object


create_task怎么样了？仅被安排一次，您何时可以使用它的用例是什么(因为它必须在事件循环已经运行的上下文中运行)？

Answer 1

任务不会被安排一次，但循环只会运行到 helloworld 完成。您会看到消息打印两次，因为循环让下一个任务运行。之后，任务停止运行，因为循环不再运行。

如果你改变

loop.run_until_complete(helloworld())

到

loop.create_task(helloworld())
loop.run_forever()

你会看到 Hello world from a coroutine! 重复打印出来。