本文介绍Python中的线程同步对象，主要涉及 thread 和 threading 模块。

threading 模块提供的线程同步原语包括：Lock、RLock、Condition、Event、Semaphore等对象。

线程执行

join与setDaemon

子线程在主线程运行结束后，会继续执行完，如果给子线程设置为守护线程(setDaemon=True)，主线程运行结束子线程即结束；

如果join()线程，那么主线程会等待子线程执行完再执行。

import threading
import time


def get_thread_a():
 print("get thread A started")
 time.sleep(3)
 print("get thread A end")


def get_thread_b():
 print("get thread B started")
 time.sleep(5)
 print("get thread B end")


if __name__ == "__main__":
 thread_a = threading.Thread(target=get_thread_a)
 thread_b = threading.Thread(target=get_thread_b)
 start_time = time.time()
 thread_b.setDaemon(True)
 thread_a.start()
 thread_b.start()
 thread_a.join()
 
 end_time = time.time()
 print("execution time: {}".format(end_time - start_time))

thread_a是join，首先子线程thread_a执行，thread_b是守护线程，当主线程执行完后，thread_b不会再执行执行结果如下：

get thread A started
get thread B started
get thread A end
execution time: 3.003199815750122

线程同步

当线程间共享全局变量，多个线程对该变量执行不同的操作时，该变量最终的结果可能是不确定的(每次线程执行后的结果不同)，如：对count变量执行加减操作 ，count的值是不确定的，要想count的值是一个确定的需对线程执行的代码段加锁。

python对线程加锁主要有Lock和Rlock模块

Lock： 

from threading import Lock
lock = Lock()
lock.acquire()
lock.release() 

Lock有acquire()和release()方法，这两个方法必须是成对出现的，acquire()后面必须release()后才能再acquire()，否则会造成死锁

Rlock：

鉴于Lock可能会造成死锁的情况，RLock(可重入锁)对Lock进行了改进，RLock可以在同一个线程里面连续调用多次acquire()，但必须再执行相同次数的release()

from threading import RLock
lock = RLock()
lock.acquire()
lock.acquire()
lock.release()
lock.release() 

condition(条件变量)，线程在执行时，当满足了特定的条件后，才可以访问相关的数据

import threading

def get_thread_a(condition):
 with condition:
  condition.wait()
  print("A : Hello B,that's ok")
  condition.notify()
  condition.wait()
  print("A : I'm fine,and you")
  condition.notify()
  condition.wait()
  print("A : Nice to meet you")
  condition.notify()
  condition.wait()
  print("A : That's all for today")
  condition.notify()

def get_thread_b(condition):
 with condition:
  print("B : Hi A, Let's start the conversation")
  condition.notify()
  condition.wait()
  print("B : How are you")
  condition.notify()
  condition.wait()
  print("B : I'm fine too")
  condition.notify()
  condition.wait()
  print("B : Nice to meet you,too")
  condition.notify()
  condition.wait()
  print("B : Oh,goodbye")

if __name__ == "__main__":
 condition = threading.Condition()
 thread_a = threading.Thread(target=get_thread_a, args=(condition,))
 thread_b = threading.Thread(target=get_thread_b, args=(condition,))
 thread_a.start()
 thread_b.start() 

Condition内部有一把锁，默认是RLock，在调用wait()和notify()之前必须先调用acquire()获取这个锁，才能继续执行；当wait()和notify()执行完后，需调用release()释放这个锁，在执行with condition时，会先执行acquire()，with结束时，执行了release()；所以condition有两层锁，最底层锁在调用wait()时会释放，同时会加一把锁到等待队列，等待notify()唤醒释放锁

wait() ：允许等待某个条件变量的通知，notify()可唤醒

notify()： 唤醒等待队列wait()

执行结果：

B : Hi A, Let's start the conversation
A : Hello B,that's ok
B : How are you
A : I'm fine,and you"htmlcode">

import time
import threading


def get_thread_a(semaphore,i):
 time.sleep(1)
 print("get thread : {}".format(i))
 semaphore.release()


def get_thread_b(semaphore):
 for i in range(10):
  semaphore.acquire()
  thread_a = threading.Thread(target=get_thread_a, args=(semaphore,i))
  thread_a.start()


if __name__ == "__main__":
 semaphore = threading.Semaphore(2)
 thread_b = threading.Thread(target=get_thread_b, args=(semaphore,))
 thread_b.start() 

上述示例了每隔1秒并发两个线程执行的情况，当调用一次semaphore.acquire()时，Semaphore的数量就减1，直至Semaphore数量为0时被锁上，当release()后Semaphore数量加1。Semaphore在本质上是调用的Condition，semaphore.acquire()在Semaphore的值为0的条件下会调用Condition.wait(), 否则将值减1，semaphore.release()会将Semaphore的值加1，并调用Condition.notify()

Semaphore源码

def acquire(self, blocking=True, timeout=None):
  if not blocking and timeout is not None:
   raise ValueError("can't specify timeout for non-blocking acquire")
  rc = False
  endtime = None
  with self._cond:
   while self._value == 0:
    if not blocking:
     break
    if timeout is not None:
     if endtime is None:
      endtime = _time() + timeout
     else:
      timeout = endtime - _time()
      if timeout <= 0:
       break
    self._cond.wait(timeout)
   else:
    self._value -= 1
    rc = True
  return rc

def release(self):
  with self._cond:
   self._value += 1
   self._cond.notify()

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