多线程(threading)示例

一、多线程简单示例

import threading,time
print('第一线程（默认）：程序开始啦！')
def takeANap():
    time.sleep(5)
    print('第二线程：5秒到，我醒来啦！')

#创建并启动多线程
t=threading.Thread(target=takeANap)
t.start()

print('第一线程（默认）：程序结束啦！')

'''
输出：
第一线程（默认）：程序开始啦！
第一线程（默认）：程序结束啦！
>>> 第二线程：5秒到，我醒来啦！
'''

二、多线程传递多参数、可选参数示例

import threading

##多线程传递多参数print('Cats','Dogs','Frogs',sep='&')=>Cats & Dogs & Frogs：
threadObj = threading.Thread(target=print, args=['Cats', 'Dogs', 'Frogs'],kwargs={'sep': ' & '})
threadObj.start()

##输出：Cats & Dogs & Frogs

三、并发问题

可以轻松地创建多个新线程，让它们同时运行。但多线程也可能会导致所谓的并发问题。如果这些线程同时读写变量，导致互相干扰，就会发生并发问题。并发问题可能很难一致地重现，所以难以调试。多线程编程本身就是一个广泛的主题。必须记住的是：为了避免并发问题，绝不让多个线程读取或写入相同的变量。当创建一个新的Thread 对象时，要确保其目标函数只使用该函数中的局部变量。这将避免程序中难以调试的并发问题。

多线程实战，多线程下载漫画：

#! python3
# multidownloadXkcd.py - Downloads XKCD comics using multiple threads.

import requests, os, bs4, threading
os.makedirs('xkcd', exist_ok=True) # store comics in ./xkcd

def downloadXkcd(startComic, endComic):
    for urlNumber in range(startComic, endComic):
        # Download the page.
        print('Downloading page http://xkcd.com/%s...' % (urlNumber))
        res = requests.get('http://xkcd.com/%s' % (urlNumber))
        res.raise_for_status()

        soup = bs4.BeautifulSoup(res.text)

        # Find the URL of the comic image.
        comicElem = soup.select('#comic img')
        if comicElem == []:
            print('Could not find comic image.')
        else:
            comicUrl = comicElem[0].get('src')
            # Download the image.
            print('Downloading image %s...' % (comicUrl))
            res = requests.get(comicUrl)
            res.raise_for_status()

            # Save the image to ./xkcd
            imageFile = open(os.path.join('xkcd', os.path.basename(comicUrl)), 'wb')
            for chunk in res.iter_content(100000):
                imageFile.write(chunk)
            imageFile.close()

# Create and start the Thread objects.
downloadThreads = [] # a list of all the Thread objects
for i in range(0, 1400, 100): # loops 14 times, creates 14 threads
    downloadThread = threading.Thread(target=downloadXkcd, args=(i, i + 99))
    downloadThreads.append(downloadThread)
    downloadThread.start()

# Wait for all threads to end.
for downloadThread in downloadThreads:
    downloadThread.join()
print('Done.')