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() 运行结果是：
-------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