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模块是比較底层的模块，python的threading模块是对thread做了一些封装，能够更加方便的被使用。可是python（cpython）因为GIL的存在无法使用threading充分利用CPU资源，假设想充分发挥多核CPU的计算能力须要使用multiprocessing模块(Windows下使用会有诸多问题)。

假设在对线程应用有较高的要求时能够考虑使用Stackless Python来完毕。Stackless Python是Python的一个改动版本号，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很多其它，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对象一个可运行方法（或对象）;另外一种是继承threading.Thread定义子类并重写run()方法。另外一种方法中，唯一必须重写的方法是run()，可依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__()必需要在函数第一行调用，否则会触发错误“AssertionError:
Thread.__init__() not called”

Python threading模块不同于其它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是彼此独立执行的线程。若想终止线程A的同一时候强力终止线程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、传输数据等）可能会没有正确的释放。所以setDaemon()并不是一个好方法,更为妥当的方式是通过Event机制。以下这段程序体现了setDaemon()和Event机制终止子线程的差别。

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()  

执行结果是：

-------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()