多线程 threading

多进程和多线程均可以执行多个任务，线程是进程的一部分。线程的特色是线程之间能够共享内存和变量，资源消耗少（不过在Unix环境中，多进程和多线程资源调度消耗差距不明显，Unix调度较快），缺点是线程之间的同步和加锁比较麻烦。

在Python中，一样能够实现多线程，有两个标准模块thread和threading，不过咱们主要使用更高级的threading模块。
1、先来看一段代码

import threading
from time import ctime,sleep
def drink(func):
    for i in range(2):
        print "I was drink %s. %s" %(func,ctime())
        sleep(1)
def eat(func):
    for i in range(2):
        print "I was eat the %s! %s" %(func,ctime())
        sleep(1)
threads = []
t1 = threading.Thread(target=drink,args=(u'牛奶',))
threads.append(t1)
t2 = threading.Thread(target=eat,args=(u'面包',))
threads.append(t2)
if __name__ == '__main__':
    for t in threads:
        t.start()
    t.join()
    print "all over %s" %ctime()

结果：

I was drink 牛奶. Wed Jan 17 14:13:06 2018
I was eat the 面包! Wed Jan 17 14:13:06 2018
I was drink 牛奶. Wed Jan 17 14:13:07 2018
I was eat the 面包! Wed Jan 17 14:13:07 2018
all over Wed Jan 17 14:13:08 2018

start是启动线程，join是阻塞当前线程，即便得在当前线程结束时，不会退出。

Python中，默认状况下，若是不加join语句，那么主线程不会等到当前线程结束才结束，但却不会当即杀死该线程。
不加join结果：

I was drink 牛奶. Wed Jan 17 14:13:23 2018
I was eat the 面包! Wed Jan 17 14:13:23 2018 all over Wed Jan 17 14:13:23 2018
I was eat the 面包! Wed Jan 17 14:13:24 2018I was drink 牛奶. Wed Jan 17 14:13:24 2018

但若是为线程实例添加t.setDaemon(True)以后，若是不加join语句，那么当主线程结束以后，会杀死子线程。

import threading
from time import ctime,sleep

def drink(func):
    for i in range(2):
        print "I was drink %s. %s" %(func,ctime())
        sleep(1)

def eat(func):
    for i in range(2):
        print "I was eat the %s! %s" %(func,ctime())
        sleep(1)

threads = []

t1 = threading.Thread(target=drink,args=(u'牛奶',))
threads.append(t1)
t2 = threading.Thread(target=eat,args=(u'面包',))
threads.append(t2)

if __name__ == '__main__':
    for t in threads:
        t.setDaemon(True)
        t.start()
    print "all over %s" %ctime()

不加join结果：

I was drink 牛奶. Wed Jan 17 14:10:43 2018
all over Wed Jan 17 14:10:43 2018I was eat the 面包! Wed Jan 17 14:10:43 2018

若是加上join,并设置等待时间，就会等待线程一段时间再退出。

import threading
from time import ctime,sleep

def drink(func):
    for i in range(2):
        print "I was drink %s. %s" %(func,ctime())
        sleep(1)

def eat(func):
    for i in range(2):
        print "I was eat the %s! %s" %(func,ctime())
        sleep(1)

threads = []

t1 = threading.Thread(target=drink,args=(u'牛奶',))
threads.append(t1)
t2 = threading.Thread(target=eat,args=(u'面包',))
threads.append(t2)

if __name__ == '__main__':
    for t in threads:
        t.setDaemon(True)
        t.start()
    t.join(0.1)
    print "all over %s" %ctime()

这里设置为0.1秒结果以下：

I was drink 牛奶. Wed Jan 17 14:11:08 2018
I was eat the 面包! Wed Jan 17 14:11:08 2018
all over Wed Jan 17 14:11:08 2018

#注释：
join()方法的位置是在for循环外的，也就是说必须等待for循环里的两个进程都结束后，才去执行主进程。