How does the scope of the global variable in threads work?

问题

I have two threads running and I want them to wait for each other at a specific line using threading.Condition(). However it seems that the global variable v is not global. It is a different variable for each thread. Here is the minimum viable product:

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()

The output is such as this:

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

but I want v to be increased to 2 by the second thread so the barrier can be passed. What is wrong?

回答1:

You can make use of the queue to make share data between threads. I am not sure if this code will work according to your use case but you can do something like this:

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()

来源：https://stackoverflow.com/questions/60185499/how-does-the-scope-of-the-global-variable-in-threads-work