python學習筆記 異步asyncio

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。html

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。編程

asyncio實現Hello world代碼以下:網絡

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,就把這個coroutine扔到eventloop中去執行併發

hello()會先打印出helloworld,而後yield from能夠讓咱們方便的調用另外一個generator,因爲asyncio.sleep(1)也是一個coroutine異步

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返回的時候,線程就在yield from拿到返回值,此處是Nonesocket

而後執行下一個語句async

把asyncio.sleep(1)當作是一個耗時1秒的IO操做。在此期間主線程沒有等待,而是去執行eventloop其餘能夠執行的coroutine所以能夠實現併發執行oop

接下來封裝2個coroutine試試網站

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()
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。

咱們用asyncio的異步網絡鏈接來獲取sina、sohu和163的網站首頁:
import asyncio

@asyncio.coroutine
def wget(host):
    print('wget %s...' % host)
    connect = asyncio.open_connection(host, 80)
    reader, writer = yield from connect
    header = 'GET / HTTP/1.0\r\nHost: %s\r\n\r\n' % host
    writer.write(header.encode('utf-8'))
    yield from writer.drain()
    while True:
        line = yield from reader.readline()
        if line == b'\r\n':
            break
        print('%s header > %s' % (host, line.decode('utf-8').rstrip()))
    # Ignore the body, close the socket
    writer.close()

loop = asyncio.get_event_loop()
tasks = [wget(host) for host in ['www.sina.com.cn', 'www.sohu.com', 'www.163.com']]
loop.run_until_complete(asyncio.wait(tasks))
loop.close()
wget www.sohu.com...
wget www.sina.com.cn...
wget www.163.com...
(等待一段時間)
(打印出sohu的header)
www.sohu.com header > HTTP/1.1 200 OK
www.sohu.com header > Content-Type: text/html
...
(打印出sina的header)
www.sina.com.cn header > HTTP/1.1 200 OK
www.sina.com.cn header > Date: Wed, 20 May 2015 04:56:33 GMT
...
(打印出163的header)
www.163.com header > HTTP/1.0 302 Moved Temporarily
www.163.com header > Server: Cdn Cache Server V2.0

asyncio提供了完善的異步IO支持;spa

異步操做須要在coroutine中經過yield from完成;

多個coroutine能夠封裝成一組Task而後併發執行。

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