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：
1 from time import ctime,sleep
2 def play_video(video):
3 for i in range(2):
4 print "i am playing video: %s at %s"%(video,ctime())
5 sleep(4)
6
7
8 def play_music(music):
9 for i in range(2):
10 print "i am playing music: %s at %s"%(music,ctime())
11 sleep(2)
12
13
14 if __name__=="__main__":
15
16 play_video("speed_and_crazy")
17 play_music("chengdu")
18
19 print "all are over at %s"%ctime()
20
21
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？
1 from time import ctime,sleep
2 import threading
3 def play_video(video):
4 for i in range(2):
5 print "i am playing video: %s at %s \n"%(video,ctime())
6 sleep(5)
7
8
9 def play_music(music):
10 for i in range(2):
11 print "i am playing music: %s at %s \n"%(music,ctime())
12 sleep(1)
13
14 threads=[]
15
16 thread1=threading.Thread(target=play_video,args=("speed_and_crazy",))
17
18 threads.append(thread1)
19
20 thread2=threading.Thread(target=play_music,args=("chengdu",))
21
22 threads.append(thread2)
23
24
25
26 if __name__=="__main__":
27
28 for thread in threads:
29 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
30 thread.start()
31
32 print "all are over at %s \n"%ctime()
33
34
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：
1 C:\Python27>python mui_thread.py 2 i am playing video: speed_and_crazy at Mon Jun 26 23:32:21 2017 3 i am playing music: chengdu at Mon Jun 26 23:32:21 2017 4 5 6 i am playing video: speed_and_crazy at Mon Jun 26 23:32:22 2017 7 8 i am playing music: chengdu at Mon Jun 26 23:32:26 2017 9 10 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
