异步不异步执行任务

Question

我正在玩Python的asyncio模块，我不知道我的简单代码有什么问题。它不会异步执行任务。

#!/usr/bin/env python3    
import asyncio
import string    

async def print_num():
    for x in range(0, 10):
        print('Number: {}'.format(x))
        await asyncio.sleep(1)    
    print('print_num is finished!')    
async def print_alp():
    my_list = string.ascii_uppercase    
    for x in my_list:
        print('Letter: {}'.format(x))
        await asyncio.sleep(1)    
    print('print_alp is finished!')    

async def msg(my_msg):
    print(my_msg)
    await asyncio.sleep(1)    

async def main():
    await msg('Hello World!')
    await print_alp()
    await msg('Hello Again!')
    await print_num()    

if __name__ == '__main__':
    loop = asyncio.get_event_loop()
    loop.run_until_complete(main())
    loop.close()

下面是调用脚本时的输出:

Hello World!
Letter: A
Letter: B
Letter: C
Letter: D
Letter: E
Letter: F
Letter: G
Letter: H
Letter: I
Letter: J
Letter: K
Letter: L
Letter: M
Letter: N
Letter: O
Letter: P
Letter: Q
Letter: R
Letter: S
Letter: T
Letter: U
Letter: V
Letter: W
Letter: X
Letter: Y
Letter: Z
print_alp is finished!
Hello Again!
Number: 0
Number: 1
Number: 2
Number: 3
Number: 4
Number: 5
Number: 6
Number: 7
Number: 8
Number: 9
print_num is finished!

Answer 1

按顺序调用函数，因此代码也按顺序执行。记住，await this的意思是"执行this，然后等待返回"(但同时，如果this选择暂停执行，其他已经在其他地方启动的任务可能会运行)。

如果您想异步运行任务，您需要:

async def main():
    await msg('Hello World!')
    task1 = asyncio.ensure_future(print_alp())
    task2 = asyncio.ensure_future(print_num())
    await asyncio.gather(task1, task2)
    await msg('Hello Again!')

参见asyncio.gather函数的文档。或者，您也可以使用asyncio.wait。

Answer 2

您正在遇到await语句的常见混淆来源，这是它们对'子' coroutines的行为顺序，但它们对'相邻' coroutines的行为异步。

例如:

import asyncio
async def child():
    i = 5
    while i > 0:
        print("Hi, I'm the child coroutine, la la la la la")
        await asyncio.sleep(1)
        i -= 1
async def parent():
    print("Hi, I'm the parent coroutine awaiting the child coroutine")
    await child() # this blocks inside the parent coroutine, but not the neighbour
    print("Hi, I'm the parent, the child coroutine is now done and I can stop waiting")
async def neighbour():
    i = 5
    while i > 0:
        await asyncio.sleep(1)
        print("Hi, I'm your neighbour!")
        i -= 1
async def my_app():
    # start the neighbour and parent coroutines and let them coexist in Task wrappers
    await asyncio.wait([neighbour(), parent()])
if __name__ == '__main__':
    loop = asyncio.get_event_loop()
    loop.run_until_complete(my_app())

将输出:

Hi, I'm the parent coroutine awaiting the child coroutine
Hi, I'm the child coroutine, la la la la la
Hi, I'm the child coroutine, la la la la la
Hi, I'm your neighbour!
Hi, I'm the child coroutine, la la la la la
Hi, I'm your neighbour!
Hi, I'm the child coroutine, la la la la la
Hi, I'm your neighbour!
Hi, I'm the child coroutine, la la la la la
Hi, I'm your neighbour!
Hi, I'm the parent, the child coroutine is now done and I can stop waiting
Hi, I'm your neighbour!
Process finished with exit code 0