Python多线程同步教程

概述

• 多线程给我们带来的好处是可以并发的执行多个任务，特别是对于I/O密集型的业务，使用多线程，可以带来成倍的性能增长。
• 可是当我们多个线程需要修改同一个数据，在不做任何同步控制的情况下，产生的结果往往是不可预料的，比如两个线程，一个输出hello，一个输出world，实际运行的结果，往往可能是一个是hello world，一个是world hello。
• python里提供了多个用于控制多线程同步的同步原语，这些原语，包含在python的标准库threading.py当中。我今天简单的介绍一下python里的这些控制多线程同步的原语，包括：Locks、RLocks、Semaphores、Events、Conditions和Barriers，你也可以继承这些类，实现自己的同步控制原语。

Lock(锁)

• Locks是python里最简单的同步原语，只包括两个状态：locked和unlocked，刚创建时状态是unlocked。Locks有两个方法，acquire和release。acquire方法加锁，release方法释放锁，如果acquire枷锁失败，则阻塞，表明其他线程已经加锁。release方法只有当状态是locked调用方法True，如果是unlocked状态，调用release方法会抛出RunTimeError异常。例如代码：

  from threading import Lock, Thread
  lock = Lock()
  g = 0
  
  def add_one():
     """
     Just used for demonstration. It’s bad to use the ‘global’
     statement in general.
     """
     global g
     lock.acquire()
     g += 1
     lock.release()
  
  def add_two():
     global g
     lock.acquire()
     g += 2
     lock.release()
  
  threads = []
  for func in [add_one, add_two]:
     threads.append(Thread(target=func))
     threads[-1].start()
  
  for thread in threads:
     """
     Waits for threads to complete before moving on with the main
     script.
     """
     thread.join()
  
  print(g)
  

• 最终输出的结果是3，通过Lock的使用，虽然在两个线程中修改了同一个全局变量，但两个线程是顺序计算出结果的。

RLock(循环锁)

• 上面的Lock对象虽然能达到同步的效果，但是无法得知当前是那个线程获取到了锁。如果锁没被释放，则其他获取这个锁的线程都会被阻塞住。如果不想阻塞，可以使用RLock，例如：

  # 使用Lock
  import threading
  num = 0
  lock = Threading.Lock()
  
  lock.acquire()
  num += 1
  lock.acquire() # 这个地方阻塞
  num += 2
  lock.release()
  
  # 使用RLock
  lock = Threading.RLock()
  lock.acquire()
  num += 3
  lock.acquire() # 这不会阻塞
  num += 4
  lock.release()
  lock.release() # 这个地方注意是释放两次锁
  

Semaphores

• Semaphores是个最简单的计数器，有两个方法acquire()和release()，如果有多个线程调用acquire()方法，acquire()方法会阻塞住，每当调用次acquire方法，就做一次减1操作，每当release()方法调用此次，就加1，如果最后的计数数值大于调用acquire()方法的线程数目，release()方法会抛出ValueError异常。下面是个生产者消费者的示例。

  import random, time
  from threading import BoundedSemaphore, Thread
  max_items = 5
  container = BoundedSemaphore(max_items)
  def producer(nloops):
      for i in range(nloops):
          time.sleep(random.randrange(2, 5))
          print(time.ctime(), end=": ")
          try:
              container.release()
              print("Produced an item.")
          except ValueError:
              print("Full, skipping.")
  def consumer(nloops):
      for i in range(nloops):
          time.sleep(random.randrange(2, 5))
          print(time.ctime(), end=": ")
          if container.acquire(False):
              print("Consumed an item.")
          else:
              print("Empty, skipping.")
  threads = []
  nloops = random.randrange(3, 6)
  print("Starting with %s items." % max_items)
  threads.append(Thread(target=producer, args=(nloops,)))
  threads.append(Thread(target=consumer, args=(random.randrange(nloops, nloops+max_items+2),)))
  for thread in threads:  # Starts all the threads.
      thread.start()
  for thread in threads:  # Waits for threads to complete before moving on with the main script.
      thread.join()
  print("All done.")
  

• threading模块还提供了一个Semaphore对象，它允许你可以任意次的调用release函数，但是最好还是使用BoundedSemaphore对象，这样在release调用次数过多时会报错，有益于查找错误。Semaphores最长用来限制资源的使用，比如最多十个进程。

Events

• event可以充当多进程之间的通信工具，基于一个内部的标志，线程可以调用set()和clear()方法来操作这个标志，其他线程则阻塞在wait()函数，直到标志被设置为True。下面的代码展示了如何利用Events来追踪行为。

  import random, time
  from threading import Event, Thread
  
  event = Event()
  
  def waiter(event, nloops):
      for i in range(nloops):
      print(“%s. Waiting for the flag to be set.” % (i+1))
      event.wait() # Blocks until the flag becomes true.
      print(“Wait complete at:”, time.ctime())
      event.clear() # Resets the flag.
      print()
  
  def setter(event, nloops):
      for i in range(nloops):
      time.sleep(random.randrange(2, 5)) # Sleeps for some time.
      event.set()
  
  threads = []
  nloops = random.randrange(3, 6)
  
  threads.append(Thread(target=waiter, args=(event, nloops)))
  threads[-1].start()
  threads.append(Thread(target=setter, args=(event, nloops)))
  threads[-1].start()
  
  for thread in threads:
      thread.join()
  
  print(“All done.”)
  

Conditions

• conditions是比events更加高级一点的同步原语，可以用户多线程间的通信和通知。比如A线程通知B线程资源已经可以被消费。其他的线程必须在调用wait()方法前调用acquire()方法。同样的，每个线程在资源使用完以后，要调用release()方法，这样其他线程就可以继续执行了。下面是使用conditions实现的一个生产者消费者的例子。

  import random, time
  from threading import Condition, Thread
  condition = Condition()
  box = []
  def producer(box, nitems):
      for i in range(nitems):
          time.sleep(random.randrange(2, 5))  # Sleeps for some time.
          condition.acquire()
          num = random.randint(1, 10)
          box.append(num)  # Puts an item into box for consumption.
          condition.notify()  # Notifies the consumer about the availability.
          print("Produced:", num)
          condition.release()
  def consumer(box, nitems):
      for i in range(nitems):
          condition.acquire()
          condition.wait()  # Blocks until an item is available for consumption.
          print("%s: Acquired: %s" % (time.ctime(), box.pop()))
          condition.release()
  threads = []
  nloops = random.randrange(3, 6)
  for func in [producer, consumer]:
      threads.append(Thread(target=func, args=(box, nloops)))
      threads[-1].start()  # Starts the thread.
  for thread in threads:
      thread.join()
  print("All done.")
  

• conditions还有其他很多用户，比如实现一个数据流API，当数据准备好了可以通知其他线程去处理数据。

Barriers

• barriers是个简单的同步原语，可以用户多个线程之间的相互等待。每个线程都调用wait()方法，然后阻塞，直到所有线程调用了wait(),然后所有线程同时开始运行。例如：

  from random import randrange
  from threading import Barrier, Thread
  from time import ctime, sleep
  
  num = 4
  b = Barrier(num)
  names = [“Harsh”, “Lokesh”, “George”, “Iqbal”]
  
  def player():
      name = names.pop()
      sleep(randrange(2, 5))
      print(“%s reached the barrier at: %s” % (name, ctime()))
      b.wait()
  
  threads = []
  print(“Race starts now…”)
  
  for i in range(num):
      threads.append(Thread(target=player))
      threads[-1].start()
  for thread in threads:
      thread.join()
  print()
  print(“Race over!”)
  

总结

• 多线程同步，说难也难，说不难也很容易，关键是要看你的业务场景和解决问题的思路，尽量降低多线程之间的依赖，理清楚业务流程，选择合适的方法，则事尽成。

• 转载自我的博客：捕蛇者说