运行多个线程类似于同时运行多个不同的程序,但具有以下优点 :
多个线程在一个进程内与主线程共享相同的数据空间,因此可以比它们是单独的进程更容易地共享信息或相互通信.
线程有时称为轻量级进程,它们不需要太多内存开销;它们比流程便宜.
线程有一个开头,一个执行序列和一个结论.它有一个指令指针,可以跟踪当前运行的上下文.
它可以被抢占(中断).
当其他线程正在运行时,它可以暂时被搁置(也称为休眠) - 这称为让步.
有两种不同类型的线程 :
内核线程
用户线程
内核线程是操作系统的一部分,而用户空间线程没有在内核中实现.
有两个模块支持Python3中的线程使用 :
_thread
threading
线程模块已被"弃用"了很长时间.建议用户使用线程模块.因此,在Python 3中,模块"thread"不再可用.但是,它已经被重命名为"_thread",用于Python3中的向后兼容性.
开始新线程
要生成另一个线程,需要在线程模块中调用以下方法 :
_thread.start_new_thread ( function, args[, kwargs] )
此方法调用可以快速有效地在Linux和Windows中创建新线程.
方法调用立即返回,子线程启动并使用传递的列表调用函数 args 当函数返回时,线程终止.
这里, args 是一个参数元组;使用空元组来调用函数而不传递任何参数. kwargs 是关键字参数的可选字典.
示例
#!/usr/bin/python3import _threadimport time# Define a function for the threaddef print_time( threadName, delay): count = 0 while count < 5: time.sleep(delay) count += 1 print ("%s: %s" % ( threadName, time.ctime(time.time()) ))# Create two threads as followstry: _thread.start_new_thread( print_time, ("Thread-1", 2, ) ) _thread.start_new_thread( print_time, ("Thread-2", 4, ) )except: print ("Error: unable to start thread")while 1: pass
输出
执行上述代码时,会产生以下结果 :
Thread-1: Fri Feb 19 09:41:39 2016Thread-2: Fri Feb 19 09:41:41 2016Thread-1: Fri Feb 19 09:41:41 2016Thread-1: Fri Feb 19 09:41:43 2016Thread-2: Fri Feb 19 09:41:45 2016Thread-1: Fri Feb 19 09:41:45 2016Thread-1: Fri Feb 19 09:41:47 2016Thread-2: Fri Feb 19 09:41:49 2016Thread-2: Fri Feb 19 09:41:53 2016
程序进入无限循环.您必须按ctrl-c才能停止
虽然它对于低级线程非常有效,但线程模块与较新的线程相比非常有限模块.
线程模块
Python 2.4中包含的新线程模块为线程提供了比线程模块更强大,更高级的支持在上一节中讨论过.
线程模块公开了线程模块的所有方法,并提供了一些额外的方法 :
threading.activeCount() : 返回活动的线程对象的数量.
threading.currentThread() : 返回调用者线程控件中线程对象的数量.
threading.enumerate() : 返回当前活动的所有线程对象的列表.
除了方法之外,线程模块还有线程实现线程的类. Thread 类提供的方法如下 :
run( ) : run()方法是线程的入口点.
start() : start()方法通过调用run方法启动一个线程.
join([time]) : join()等待线程终止.
isAlive() : isAlive()方法检查线程是否仍在执行.
getName() : getName()方法返回一个线程的名称.
setName() : setName()方法设置线程的名称.
使用线程模块创建线程
要使用线程模块实现新线程,您必须执行以下操作 :
定义新的子类线程类.
覆盖 __ init __(self [,args])方法以添加额外的参数.
然后,重写run(self [,args])方法以实现线程在启动时应该执行的操作.
创建新的 Thread 子类后,可以创建它的实例,然后通过调用
示例
#!/usr/bin/python3import threadingimport timeexitFlag = 0class myThread (threading.Thread): def __init__(self, threadID, name, counter): threading.Thread.__init__(self) self.threadID = threadID self.name = name self.counter = counter def run(self): print ("Starting " + self.name) print_time(self.name, self.counter, 5) print ("Exiting " + self.name)def print_time(threadName, delay, counter): while counter: if exitFlag: threadName.exit() time.sleep(delay) print ("%s: %s" % (threadName, time.ctime(time.time()))) counter -= 1# Create new threadsthread1 = myThread(1, "Thread-1", 1)thread2 = myThread(2, "Thread-2", 2)# Start new Threadsthread1.start()thread2.start()thread1.join()thread2.join()print ("Exiting Main Thread")
结果
当我们运行上述程序时,它会产生以下结果 :
Starting Thread-1Starting Thread-2Thread-1: Fri Feb 19 10:00:21 2016Thread-2: Fri Feb 19 10:00:22 2016Thread-1: Fri Feb 19 10:00:22 2016Thread-1: Fri Feb 19 10:00:23 2016Thread-2: Fri Feb 19 10:00:24 2016Thread-1: Fri Feb 19 10:00:24 2016Thread-1: Fri Feb 19 10:00:25 2016Exiting Thread-1Thread-2: Fri Feb 19 10:00:26 2016Thread-2: Fri Feb 19 10:00:28 2016Thread-2: Fri Feb 19 10:00:30 2016Exiting Thread-2Exiting Main Thread
同步线程
Python提供的线程模块包含一个易于实现的锁定机制允许您同步线程.通过调用 Lock()方法创建一个新锁,该方法返回新锁.
获取(阻塞)方法新的锁对象用于强制线程同步运行.可选的 blocking 参数使您可以控制线程是否等待获取锁定.
如果阻止设置为0,则如果无法获取锁,则线程立即返回0值,如果获取了锁,则返回1.如果阻塞设置为1,则线程会阻塞并等待锁被释放.
新锁对象的 release()方法用于在不再需要时释放锁.
示例
#!/usr/bin/python3import threadingimport timeclass myThread (threading.Thread): def __init__(self, threadID, name, counter): threading.Thread.__init__(self) self.threadID = threadID self.name = name self.counter = counter def run(self): print ("Starting " + self.name) # Get lock to synchronize threads threadLock.acquire() print_time(self.name, self.counter, 3) # Free lock to release next thread threadLock.release()def print_time(threadName, delay, counter): while counter: time.sleep(delay) print ("%s: %s" % (threadName, time.ctime(time.time()))) counter -= 1threadLock = threading.Lock()threads = []# Create new threadsthread1 = myThread(1, "Thread-1", 1)thread2 = myThread(2, "Thread-2", 2)# Start new Threadsthread1.start()thread2.start()# Add threads to thread listthreads.append(thread1)threads.append(thread2)# Wait for all threads to completefor t in threads: t.join()print ("Exiting Main Thread")
输出
执行上述代码时,会产生以下结果 :
Starting Thread-1Starting Thread-2Thread-1: Fri Feb 19 10:04:14 2016Thread-1: Fri Feb 19 10:04:15 2016Thread-1: Fri Feb 19 10:04:16 2016Thread-2: Fri Feb 19 10:04:18 2016Thread-2: Fri Feb 19 10:04:20 2016Thread-2: Fri Feb 19 10:04:22 2016Exiting Main Thread
多线程优先级队列
队列模块允许您创建一个可以容纳特定号码的新队列对象的项目.有以下方法来控制队列和减号;
get() : get()从队列中删除并返回一个项目.
put() : put将项目添加到队列中.
qsize() : qsize()返回当前队列中的项目数.
empty() : 如果队列为空,则empty()返回True;否则,错误.
full() : 如果队列已满,则full()返回True;否则,错误.
示例
#!/usr/bin/python3import queueimport threadingimport timeexitFlag = 0class myThread (threading.Thread): def __init__(self, threadID, name, q): threading.Thread.__init__(self) self.threadID = threadID self.name = name self.q = q def run(self): print ("Starting " + self.name) process_data(self.name, self.q) print ("Exiting " + self.name)def process_data(threadName, q): while not exitFlag: queueLock.acquire() if not workQueue.empty(): data = q.get() queueLock.release() print ("%s processing %s" % (threadName, data)) else: queueLock.release() time.sleep(1)threadList = ["Thread-1", "Thread-2", "Thread-3"]nameList = ["One", "Two", "Three", "Four", "Five"]queueLock = threading.Lock()workQueue = queue.Queue(10)threads = []threadID = 1# Create new threadsfor tName in threadList: thread = myThread(threadID, tName, workQueue) thread.start() threads.append(thread) threadID += 1# Fill the queuequeueLock.acquire()for word in nameList: workQueue.put(word)queueLock.release()# Wait for queue to emptywhile not workQueue.empty(): pass# Notify threads it's time to exitexitFlag = 1# Wait for all threads to completefor t in threads: t.join()print ("Exiting Main Thread")
输出
当执行上面的代码时,它产生以下结果 :
Starting Thread-1Starting Thread-2Starting Thread-3Thread-1 processing OneThread-2 processing TwoThread-3 processing ThreeThread-1 processing FourThread-2 processing FiveExiting Thread-3Exiting Thread-1Exiting Thread-2Exiting Main Thread