python异步编程案例之协程嵌套

协程的嵌套，意思就是在一个协程中await另外一个协程。要实现这个目标，我们要将协程封装成Task, 因为其他代码可以直接await这个Task。
下面代码嵌套关系是： main协程嵌套evaluate_work协程，evaluate_work协程嵌套do_some_work协程。启动事件循环后，先执行main, 进入for loop, 然后 await evaluate_work(task)会执行evaluate_work协程(不明白协程如何执行，请参考我的另一篇博客，之协程执行)。

import time
import asyncio
now = lambda: time.time()

async def do_some_work(x):
    print("do_some_work: ", x)
    return "some_work is done for {}".format(x)
    
async def evaluate_work(task):
    print('evaluation: starting...')
    result = await task
    print("work evaluation returned: ", result)
    
async def main():
    for i in range(5):
        work_coroutine = do_some_work(i)
        # task = loop.create_task(work_coroutine )
        task = asyncio.ensure_future(work_coroutine)
        await evaluate_work(task)
        
start = now()
loop = asyncio.new_event_loop()
asyncio.set_event_loop(loop)
loop.run_until_complete(main())
loop.close()
end = now()
print("total run time:", end - start)

程序是这样执行得：启动事件循环，执行main函数，main函数里进入for loop。创建一个work_coroutine, 并封装为task。task作为参数传入evaluate_work里， 并执行这个evaluate_work协程对象。进入evaluate_work， 先打印print(‘evaluation: starting…’)，又遇到 await task（ 这里的task 参数是传入得work_coroutine协程）于是执行do_some_work，进入do_some_work, 先打印print("do_some_work: “, x)，然后返回"some_work is done for {}”.format(x)。紧接着回到evaluate_work中，把返回得结果赋值给result, 接着打印print("work evaluation returned: ", result)。如此重复for loop, 直到执行完为止。

evaluation: starting...
do_some_work:  0
work evaluation returned:  some_work is done for 0
evaluation: starting...
do_some_work:  1
work evaluation returned:  some_work is done for 1
evaluation: starting...
do_some_work:  2
work evaluation returned:  some_work is done for 2
evaluation: starting...
do_some_work:  3
work evaluation returned:  some_work is done for 3
evaluation: starting...
do_some_work:  4
work evaluation returned:  some_work is done for 4
total run time: 0.0027039051055908203

在main函数里，再加一个asyncio.gather 或者 asyncio.wait方法，都可以再次收集协程return的结果。

import time
import asyncio
now = lambda: time.time()
async def do_some_work(x):
    print("do_some_work: ", x)
    return "some_work is done for {}".format(x)
async def evaluate_work(task):
    print('evaluation: starting...')
    result = await task
    print("work evaluation returned: ", result)
async def main():
    tasks = []
    for i in range(5):
        work_coroutine = do_some_work(i)
        task = asyncio.ensure_future(work_coroutine)
        tasks.append(task)
        # task = loop.create_task(coroutine)
        await evaluate_work(task)
    # dones, pendings = await asyncio.wait(tasks)
    # for task in dones:
    #     print("task result:", task.result())
    results = await asyncio.gather(*tasks)
    for result in results:
        print("Task result:", result)
start = now()
loop = asyncio.new_event_loop()
asyncio.set_event_loop(loop)
loop.run_until_complete(main())
loop.close()
end = now()
print("total run time:", end - start)

evaluation: starting...
do_some_work:  0
work evaluation returned:  some_work is done for 0
evaluation: starting...
do_some_work:  1
work evaluation returned:  some_work is done for 1
evaluation: starting...
do_some_work:  2
work evaluation returned:  some_work is done for 2
evaluation: starting...
do_some_work:  3
work evaluation returned:  some_work is done for 3
evaluation: starting...
do_some_work:  4
work evaluation returned:  some_work is done for 4
Task result: some_work is done for 0
Task result: some_work is done for 1
Task result: some_work is done for 2
Task result: some_work is done for 3
Task result: some_work is done for 4
total run time: 0.0023107528686523438