Python多线程之线程创建和终止

python主要是通过thread和threading这两个模块来实现多线程支持。Python的thread模块是比较底层的模块，python的threading模块是对thread做了一些封装，可以更加方便的被使用。但是python（cpython）由于GIL的存在无法使用threading充分利用CPU资源，如果想充分发挥多核CPU的计算能力需要使用multiprocessing模块(windows下使用会有诸多问题)。

如果在对线程应用有较高的要求时可以考虑使用Stackless Python来完成。Stackless Python是Python的一个修改版本，对多线程编程有更好的支持，提供了对微线程的支持。微线程是轻量级的线程，在多个线程间切换所需的时间更多，占用资源也更少。

通过threading模块创建新的线程有两种方法：一种是通过threading.Thread(Target=executable Method)-即传递给Thread对象一个可执行方法（或对象）;第二种是继承threading.Thread定义子类并重写run()方法。第二种方法中，唯一必须重写的方法是run()，可根据需要决定是否重写__init__()。值得注意的是，若要重写__init__()，父类的__init__()必须要在函数第一行调用，否则会触发错误“AssertionError: Thread.__init__() not called”

Python threading模块不同于其他语言之处在于它没有提供线程的终止方法，通过Python threading.Thread()启动的线程彼此是独立的，若在线程A中启动了线程B，那么A、B是彼此独立运行的线程。若想终止线程A的同时强力终止线程B，一个简单的方法是通过在线程A中调用B.setDaemon(True)实现。但这样带来的问题是：线程B中的资源（打开的文件、数据传输等）可能会没有正确的释放。所以setDaemon()并非一个好方法,更为妥当的方式是通过Event机制。下面这段程序体现了setDaemon()和Event机制终止子线程的区别。

import threading  
import time  
class mythread(threading.Thread):   
    def __init__(self,stopevt = None,File=None,name = 'subthread',Type ='event'):   
        threading.Thread.__init__(self)   
        self.stopevt = stopevt   
        self.name = name   
        self.File = File   
        self.Type = Type   
           
                  
    def Eventrun(self):   
        while not self.stopevt.isSet():   
            print self.name +' alive\n'   
            time.sleep(2)   
        if self.File:   
            print 'close opened file in '+self.name+'\n'   
            self.File.close()   
        print self.name +' stoped\n'   
       
    def Daemonrun(self):   
        D = mythreadDaemon(self.File)   
        D.setDaemon(True)   
        while not self.stopevt.isSet():   
            print self.name +' alive\n'   
            time.sleep(2)   
        print self.name +' stoped\n'   
    def run(self):   
        if self.Type == 'event': self.Eventrun()   
        else: self.Daemonrun()   
class mythreadDaemon(threading.Thread):   
    def __init__(self,File=None,name = 'Daemonthread'):   
        threading.Thread.__init__(self)   
        self.name = name   
        self.File = File   
    def run(self):   
        while True:   
            print self.name +' alive\n'   
            time.sleep(2)   
        if self.File:   
            print 'close opened file in '+self.name+'\n'   
            self.File.close()   
        print self.name +' stoped\n'   
           
def evtstop():   
    stopevt = threading.Event()   
    FileA = open('testA.txt','w')   
    FileB = open('testB.txt','w')   
    A = mythread(stopevt,FileA,'subthreadA')   
    B = mythread(stopevt,FileB,'subthreadB')   
    print repr(threading.currentThread())+'alive\n'   
    print FileA.name + ' closed? '+repr(FileA.closed)+'\n'   
    print FileB.name + ' closed? '+repr(FileB.closed)+'\n'   
    A.start()   
    B.start()   
    time.sleep(1)   
    print repr(threading.currentThread())+'send stop signal\n'   
    stopevt.set()   
    A.join()   
    B.join()   
    print  repr(threading.currentThread())+'stoped\n'   
    print 'after A stoped, '+FileA.name + ' closed? '+repr(FileA.closed)+'\n'   
    print 'after A stoped, '+FileB.name + ' closed? '+repr(FileB.closed)+'\n'   
def daemonstop():   
    stopevt = threading.Event()   
    FileA = open('testA.txt','r')   
    A = mythread(stopevt,FileA,'subthreadA',Type = 'Daemon')   
    print repr(threading.currentThread())+'alive\n'   
    print FileA.name + ' closed? '+repr(FileA.closed)+'\n'   
    A.start()   
    time.sleep(1)   
    stopevt.set()   
    A.join()   
    print  repr(threading.currentThread())+'stoped\n'   
    print 'after A stoped, '+FileA.name + ' closed? '+repr(FileA.closed)+'\n'   
    if not FileA.closed:   
        print 'You see the differents, the resource in subthread may not released with setDaemon()'   
        FileA.close()   
if __name__ =='__main__':   
    print '-------stop subthread example with Event:----------\n'   
    evtstop()   
    print '-------Daemon stop subthread example :----------\n'   
    daemonstop()  

-------stop subthread example with Event:----------   
<_MainThread(MainThread, started 2436)>alive   
testA.txt closed? False  
testB.txt closed? False  
subthreadA alive   
subthreadB alive   
  
<_MainThread(MainThread, started 2436)>send stop signal  
close opened file in subthreadA   
close opened file in subthreadB   
  
subthreadA stoped   
subthreadB stoped   
  
<_MainThread(MainThread, started 2436)>stoped   
after A stoped, testA.txt closed? True  
after A stoped, testB.txt closed? True  
-------Daemon stop subthread example :----------   
<_MainThread(MainThread, started 2436)>alive   
testA.txt closed? False  
subthreadA alive   
subthreadA stoped   
<_MainThread(MainThread, started 2436)>stoped   
after A stoped, testA.txt closed? False  
You see the differents, the resource in subthread may not released with setDaemon()