线程中全局变量的作用域如何工作？

Question

我有两个正在运行的线程，我希望它们使用threading.Condition（）在特定的行上互相等待。但是，似乎全局变量v不是全局变量。每个线程的变量都不同。这是最低可行的产品：

import threading
lock = threading.Condition()
v = 0
n = 2

def barrier():
    with lock:
        global v
        v =+ 1
        print("v is increased to " + str(v))
        if v == n:
            print("v is equal to n")
            lock.notifyAll()
            print("v equals n")
            v = 0
        else:
            lock.wait()




class ServerThread(threading.Thread):
    def __init__(self):
        threading.Thread.__init__(self)

    def run(self):
        print("before barrier")
        barrier()
        print("after barrier")



for i in range(2):
            t = ServerThread()
            t.start()

输出是这样的：

before barrier
v is increased to 1
before barrier
v is increased to 1

但是我希望第二个线程将v增加到2，以便可以传递障碍。怎么了？

Answer 1

您可以利用队列在线程之间共享数据。我不确定此代码是否会根据您的用例工作，但是您可以执行以下操作：

import threading, queue
lock = threading.Condition()
q = queue.Queue()
q.put(0)
n = 2

def barrier():
    with lock:
        v_increase = q.get() + 1
        q.put(v_increase)
        print("v is increased to " + str(v_increase))
        if v_increase == n:
            print("v is equal to n")
            lock.notifyAll()
            print("v equals n")
            q.get()
            q.put(0)
        else:
            lock.wait()




class ServerThread(threading.Thread):
    def __init__(self):
        threading.Thread.__init__(self)

    def run(self):
        global v
        print("before barrier")
        barrier()
        print("after barrier")



for i in range(2):
            t = ServerThread()
            t.start()