python | task

task 就是任务，即可以向事件循环中添加或者注册任务，等待事件循环执行。

task 通过 asyncio.create_task() 「python3.7 引入」 创建，也可以通过

loop = asyncio.get_event_loop()
loop.create_task()

创建，还可以通过 asyncio.ensure_future() 「python3.7 之前的用法」创建。

import asyncio


async def run0():
    await asyncio.sleep(1)
    print("run0")


async def main():
    task1 = asyncio.create_task(run0())
    task2 = asyncio.create_task(run0())

    r1 = await task1
    r2 = await task2

    print(r1, r2)


asyncio.run(main())

优化成

import asyncio


async def run0():
    await asyncio.sleep(1)
    print("run0")


async def main():
    tasks = [asyncio.create_task(run0()),
             asyncio.create_task(run0())]

    done, pending = await asyncio.wait(tasks)
    print(done)


asyncio.run(main())

输出

run0
run0
{<Task finished coro=<run0() done, defined at /Users/licong/python/chengzi/baojia/info/test.py:4> result=None>, <Task finished coro=<run0() done, defined at /Users/licong/python/chengzi/baojia/info/test.py:4> result=None>}

先记住一点，事件循环是在

asyncio.run(main())

创建的，如果改成

import asyncio


async def run0():
    await asyncio.sleep(1)
    print("run0")

tasks = [asyncio.create_task(run0()),
         asyncio.create_task(run0())]

asyncio.run(asyncio.wait(tasks))

会报错

RuntimeError: no running event loop
sys:1: RuntimeWarning: coroutine 'run0' was never awaited

因为执行

asyncio.create_task(run0())

的时候，会把 task 放到事件循环中，但是，此时事件循环还没有创建，变成下面这样就好了

import asyncio


async def run0():
    await asyncio.sleep(1)
    print("run0")


tasks = [run0(), run0()]

asyncio.run(asyncio.wait(tasks))

因为数组里放的是协程对象。