Python的多线程编程

提到多线程，很多人就会望而却步，本文将由浅入深地带你攻克python多线程编程，并防止你跳入深坑，

首先看一段简单的代码：

 from time import ctime,sleep
 def play_video(video):
     for i in range(2):
         print "i am playing video: %s at %s"%(video,ctime())
         sleep(4)

 def play_music(music):
     for i in range(2):
         print "i am playing music: %s at %s"%(music,ctime())
         sleep(2)

 if __name__=="__main__":

     play_video("speed_and_crazy")
     play_music("chengdu")

     print "all are over at %s"%ctime()

执行结果：

C:\Python27>python mui_thread.py
i am playing video: speed_and_crazy at Mon Jun 26 23:01:59 2017
i am playing video: speed_and_crazy at Mon Jun 26 23:02:03 2017
i am playing music: chengdu at Mon Jun 26 23:02:07 2017
i am playing music: chengdu at Mon Jun 26 23:02:09 2017
all are over at Mon Jun 26 23:02:11 2017

随着人们对多任务的要求，同时为了充分利用cpu资源，多线程编程不可避免，那么我们如何利用python去实现play_video和play_music

两个任务同时运行呢？

 from time import ctime,sleep
 import threading
 def play_video(video):
     for i in range(2):
         print "i am playing video: %s at %s \n"%(video,ctime())
         sleep(5)

 def play_music(music):
     for i in range(2):
         print "i am playing music: %s at %s \n"%(music,ctime())
         sleep(1)

 threads=[]

 thread1=threading.Thread(target=play_video,args=("speed_and_crazy",))

 threads.append(thread1)

 thread2=threading.Thread(target=play_music,args=("chengdu",))

 threads.append(thread2)

 if __name__=="__main__":

     for thread in threads:
         thread.setDaemon(True) #将线程声明为守护线程，必须在start()方法调用之前，如果不设置为守护线程,程序会被无限挂起
         thread.start()

     print "all are over at %s \n"%ctime()

测试结果：

C:\Python27>python mui_thread.py
i am playing video: speed_and_crazy at Mon Jun 26 23:18:52 2017
all are over at Mon Jun 26 23:18:52 2017
i am playing music: chengdu at Mon Jun 26 23:18:52 2017  #从打印的时间可知，play_video、play_music和父进程几乎同时运行

从结果看，与我们最初的目标相差甚远，怎么没有按照顺序执行，为什么每个函数都只有一条日记输出？

那是因为子线程（play_video、play_music）和主线程print "all are over at %s \n"%ctime()都是同一时间启动，但由于主线程已经运行结束，所以导致子线程也同时终止，在这种条件下，我们如何保证子进程都能够执行完毕呢？

增加thread.join()并 放在循环外

from time import ctime,sleep
import threading
def play_video(video):
    for i in range(2):
        print "i am playing video: %s at %s \n"%(video,ctime())
        sleep(1)

def play_music(music):
    for i in range(2):
        print "i am playing music: %s at %s \n"%(music,ctime())
        sleep(5)

threads=[]

thread1=threading.Thread(target=play_video,args=("speed_and_crazy",))

threads.append(thread1)

thread2=threading.Thread(target=play_music,args=("chengdu",))

threads.append(thread2)

if __name__=="__main__":

    for thread in threads:
        thread.setDaemon(True)
        thread.start()
    thread.join()  #加在循环外，
    print "all are over at %s \n"%ctime()

运行结果：

 C:\Python27>python mui_thread.py
 i am playing video: speed_and_crazy at Mon Jun 26 23:32:21 2017
 i am playing music: chengdu at Mon Jun 26 23:32:21 2017

 i am playing video: speed_and_crazy at Mon Jun 26 23:32:22 2017

 i am playing music: chengdu at Mon Jun 26 23:32:26 2017

 all are over at Mon Jun 26 23:32:31 2017

thread.join()的作用是主线程必须等待子线程都执行完了才能结束，play_video、play_music几乎同时执行
但是如果改变play_video、play_music里面的sleep的时长，即是下面的代码：

from time import ctime,sleep
import threading
def play_video(video):
    for i in range(2):
        print "i am playing video: %s at %s \n"%(video,ctime())
        sleep(5)

def play_music(music):
    for i in range(2):
        print "i am playing music: %s at %s \n"%(music,ctime())
        sleep(1)

threads=[]

thread1=threading.Thread(target=play_video,args=("speed_and_crazy",))

threads.append(thread1)

thread2=threading.Thread(target=play_music,args=("chengdu",))

threads.append(thread2)

if __name__=="__main__":

    for thread in threads:
        thread.setDaemon(True)
        thread.start()
    thread.join()
    print "all are over at %s \n"%ctime()

此时运行结果：

C:\Python27>python mui_thread.py
i am playing video: speed_and_crazy at Mon Jun 26 23:44:13 2017
i am playing music: chengdu at Mon Jun 26 23:44:13 2017

i am playing music: chengdu at Mon Jun 26 23:44:14 2017

all are over at Mon Jun 26 23:44:15 2017

我们看到play_video还有一条log没有打印出来，原因是thread.join()在循环外，此时的thread为play_music，父进程只会等待play_music进程执行完就结束，而不会等待play_video（sleep时间较长）执行完才结束，所以才会有上面的结果