Python asyncio 模塊

Python 3.4

asyncio是Python 3.4版本引入的標準庫，直接內置了對異步IO的支持。

asyncio的編程模型就是一個消息循環。咱們從asyncio模塊中直接獲取一個EventLoop的引用，而後把須要執行的協程扔到EventLoop中執行，就實現了異步IO。

用asyncio實現Hello world代碼以下：

import asyncio

@asyncio.coroutine
def hello():
    print("Hello world!")
    # 異步調用asyncio.sleep(1):
    r = yield from asyncio.sleep(1)
    print("Hello again!")

# 獲取EventLoop:
loop = asyncio.get_event_loop()
# 執行coroutine
loop.run_until_complete(hello())
loop.close()

@asyncio.coroutine把一個generator標記爲coroutine類型，而後，咱們就把這個coroutine扔到EventLoop中執行。

hello()會首先打印出Hello world!，而後，yield from語法可讓咱們方便地調用另外一個generator。因爲asyncio.sleep()也是一個coroutine，因此線程不會等待asyncio.sleep()，而是直接中斷並執行下一個消息循環。當asyncio.sleep()返回時，線程就能夠從yield from拿到返回值（此處是None），而後接着執行下一行語句。

把asyncio.sleep(1)當作是一個耗時1秒的IO操做，在此期間，主線程並未等待，而是去執行EventLoop中其餘能夠執行的coroutine了，所以能夠實現併發執行。



咱們用Task封裝兩個coroutine試試：

import threading
import asyncio

@asyncio.coroutine
def hello():
    print('Hello world! (%s)' % threading.currentThread())
    yield from asyncio.sleep(1)
    print('Hello again! (%s)' % threading.currentThread())

loop = asyncio.get_event_loop()
tasks = [hello(), hello()]
loop.run_until_complete(asyncio.wait(tasks))
loop.close()
觀察執行過程：

Hello world! (<_MainThread(MainThread, started 140735195337472)>)
Hello world! (<_MainThread(MainThread, started 140735195337472)>)
(暫停約1秒)
Hello again! (<_MainThread(MainThread, started 140735195337472)>)
Hello again! (<_MainThread(MainThread, started 140735195337472)>)
由打印的當前線程名稱能夠看出，兩個coroutine是由同一個線程併發執行的。

若是把asyncio.sleep()換成真正的IO操做，則多個coroutine就能夠由一個線程併發執行。

參考：https://www.liaoxuefeng.com/wiki/1016959663602400/1017970488768640

python3.5


關於asyncio的一些關鍵字的說明：

event_loop 事件循環：程序開啓一個無限循環，把一些函數註冊到事件循環上，當知足事件發生的時候，調用相應的協程函數

coroutine 協程：協程對象，指一個使用async關鍵字定義的函數，它的調用不會當即執行函數，而是會返回一個協程對象。協程對象須要註冊到事件循環，由事件循環調用。

task 任務：一個協程對象就是一個原生能夠掛起的函數，任務則是對協程進一步封裝，其中包含了任務的各類狀態

future: 表明未來執行或沒有執行的任務的結果。它和task上沒有本質上的區別

async/await 關鍵字：python3.5用於定義協程的關鍵字，async定義一個協程，await用於掛起阻塞的異步調用接口。

import asyncio
import time

now = lambda :time.time()

async def do_some_work(x):
    print("waiting:",x)
    # await 後面就是調用耗時的操做
    await asyncio.sleep(x)
    return "Done after {}s".format(x)

start = now()

coroutine = do_some_work(2)
loop = asyncio.get_event_loop()
task = asyncio.ensure_future(coroutine)
loop.run_until_complete(task)

print("Task ret:", task.result())
print("Time:", now() - start)

參考：https://www.cnblogs.com/zhaof/p/8490045.html

Python3.7

傳統的asyncio異步事件循環
在Python3.7之前的版本，調用異步函數前要先調用asyncio.get_event_loop()函數獲取事件循環loop對象，而後經過不一樣的策略調用loop.run_forever()方法或者loop.run_until_complete()方法執行異步函數。一個典型的例子是這樣的。

import asyncio
import random
import datetime

async def wait_and_echo(content):
    wait = random.randint(0, 10)
    print(f'print {content} after {wait} seconds')
    await asyncio.sleep(wait)
    print(f'{content} printed at {datetime.datetime.utcnow().strftime("%H:%M:%S ")}')

async def main():
    await asyncio.gather(*[wait_and_echo(x) for x in range(10)])

loop = asyncio.get_event_loop()
tasks = [wait_and_echo(x) for x in range(10)]
loop.run_until_complete(asyncio.gather(*tasks))
運行結果如：

print 0 after 9 seconds
print 1 after 0 seconds
print 2 after 7 seconds
print 3 after 2 seconds
print 4 after 8 seconds
print 5 after 3 seconds
print 6 after 2 seconds
print 7 after 9 seconds
print 8 after 1 seconds
print 9 after 1 seconds
1 printed at 05:15:26
8 printed at 05:15:27
9 printed at 05:15:27
6 printed at 05:15:28
3 printed at 05:15:28
5 printed at 05:15:29
2 printed at 05:15:33
4 printed at 05:15:34
0 printed at 05:15:35
7 printed at 05:15:35
使用asyncio.run()函數執行異步函數
asyncio.run()函數的官方文檔是這樣子的:

Signature: asyncio.run(main, *, debug=False)
Docstring:
Run a coroutine.

This function runs the passed coroutine, taking care of
managing the asyncio event loop and finalizing asynchronous
generators.

This function cannot be called when another asyncio event loop is
running in the same thread.

If debug is True, the event loop will be run in debug mode.

This function always creates a new event loop and closes it at the end.
It should be used as a main entry point for asyncio programs, and should
ideally only be called once.

Example:

    async def main():
        await asyncio.sleep(1)
        print('hello')

    asyncio.run(main())
File:      c:\users\pc\appdata\local\programs\python\python37\lib\asyncio\runners.py
Type:      function
使用Python3.7中的新APIasyncio.run(),上述例子能夠改寫爲：

import asyncio
import random
import datetime

async def wait_and_echo(content):
    wait = random.randint(0, 10)
    print(f'print {content} after {wait} seconds')
    await asyncio.sleep(wait)
    print(f'{content} printed at {datetime.datetime.utcnow().strftime("%H:%M:%S ")}')

async def main():
    await asyncio.gather(*[wait_and_echo(x) for x in range(10)])

asyncio.run(main())
運行結果並無差別。