asyncio标准库1 Hello World

利用asyncio的event loop，编写和调度协程
coroutine [,kəuru:'ti:n] n. 协程

Simple coroutine（调用1个协程）

import asyncio

async def say(what, when):
    await asyncio.sleep(when)
    print(what)

loop = asyncio.get_event_loop()
loop.run_until_complete(say('hello world', 1))  # 使用run_until_complete()方法，在协程完成后中断event loop。
loop.close()

Creating tasks（调用多个协程）

import asyncio

async def say(what, when):
    await asyncio.sleep(when)
    print(what)

loop = asyncio.get_event_loop()

loop.create_task(say('first hello', 2))
loop.create_task(say('second hello', 1))

loop.run_forever()  # 使用run_forever()方法，协程会一直运行，不会中断event loop
loop.close()

Stopping the loop

import asyncio

async def say(what, when):
    await asyncio.sleep(when)
    print(what)

async def stop_after(loop, when):
    await asyncio.sleep(when)
    loop.stop()  # 中断event loop

loop = asyncio.get_event_loop()

loop.create_task(say('first hello', 2))
loop.create_task(say('second hello', 1))
loop.create_task(say('third hello', 4))
loop.create_task(stop_after(loop, 3))

loop.run_forever()
loop.close()

# out:
second hello
first hello
Task was destroyed but it is pending!
task: <Task pending coro=<say() done, defined at e03.py:5> wait_for=<Future pending cb=[<TaskWakeupMethWrapper object at 0x7fed59595a68>()]>>

# 在执行2个任务后，中断event loop，'third hello‘任务由于延迟时间4秒，未能执行。