如何将子线程变量传递至主线程并实现自动重置？

能否将子线程thread1中task函数内的变量a、b传递至主线程thread0的while循环中？要求主线程使用一次这些变量后，自动将其重置为0。尝试将变量设为全局变量但未生效，这种场景下能否对变量使用Lock？

尝试代码 #

# example of running a function in another thread
from threading import Thread, Lock
import time

global a
global b
# setting some initial variables (needed?)
a = 0
b = 0


# a custom function that will run commands for new thread ("thread1")
def task():
    while True:
        global a
        global b
        
        #lets reset the values to ensure they are 0
        a = 0
        b = 0


        print "This is the TASK function.\n"
        a = int(raw_input("Enter number: "))
        b  = int(a * 5.6)
        print "I received: ", a
        print "I calculated: ", b




# create & run a new thread ("thread1")
thread = Thread(target=task)
thread.daemon = True
thread.start()


while True:
    print "-------------------------------"
    print "HERE I'm running the MAIN thread (thread0)."


    print "DEGREES received from TASK function: ", a
    print "COUNTS received from TASK function: ", b
    time.sleep(1)

预期输出 #

python ./thread2.py 
This is the TASK function.

Enter number: -------------------------------
HERE I'm running the MAIN thread (thread0).
DEGREES received from TASK function:  0
COUNTS received from TASK function:  0
-------------------------------
HERE I'm running the MAIN thread (thread0).
DEGREES received from TASK function:  0
COUNTS received from TASK function:  0
5     (**** note this is the number entered by the user****)
I received:  5
I calculated:  28
This is the TASK function.
-------------------------------
HERE I'm running the MAIN thread (thread0).
DEGREES received from TASK function:  5
COUNTS received from TASK function:  28
-------------------------------
HERE I'm running the MAIN thread (thread0).
DEGREES received from TASK function:  0
COUNTS received from TASK function:  0
-------------------------------
Enter number:
HERE I'm running the MAIN thread (thread0).
DEGREES received from TASK function:  0
COUNTS received from TASK function:  0
-------------------------------
HERE I'm running the MAIN thread (thread0).
DEGREES received from TASK function:  0
COUNTS received from TASK function:  0
8     (**** note this is the number entered by the user****)
I received:  8
I calculated:  45
This is the TASK function.
Enter number: -------------------------------
HERE I'm running the MAIN thread (thread0).
DEGREES received from TASK function:  8
COUNTS received from TASK function:  45
-------------------------------
HERE I'm running the MAIN thread (thread0).
DEGREES received from TASK function:  0
COUNTS received from TASK function:  0
-------------------------------
HERE I'm running the MAIN thread (thread0).
DEGREES received from TASK function:  0
COUNTS received from TASK function:  0
-------------------------------

问题根源 #

你之前的代码里，子线程task的while循环每次启动都会立刻把a、b重置为0，而且主线程和子线程读写全局变量时没有同步，导致主线程要么读不到新值，要么刚读到就被子线程重置，所以全局变量看似“没生效”。

关于Lock的使用 #

这种场景必须用Lock，多线程共享变量时，读写操作需要保证原子性，否则会出现竞态条件（比如主线程读取到一半，子线程修改了变量值）。

修改后的代码 #

from threading import Thread, Lock
import time

# 初始化全局变量和锁
a = 0
b = 0
lock = Lock()

def task():
    global a, b
    while True:
        print "This is the TASK function.\n"
        # 获取用户输入并计算
        input_num = int(raw_input("Enter number: "))
        calc_num = int(input_num * 5.6)
        print "I received: ", input_num
        print "I calculated: ", calc_num
        
        # 加锁更新全局变量
        with lock:
            a = input_num
            b = calc_num
        
        # 子线程不需要主动重置，交给主线程处理

# 创建并启动子线程
thread = Thread(target=task)
thread.daemon = True
thread.start()

while True:
    print "-------------------------------"
    print "HERE I'm running the MAIN thread (thread0)."
    
    # 加锁读取并重置变量
    with lock:
        current_a = a
        current_b = b
        # 读取后立即重置为0
        a = 0
        b = 0
    
    print "DEGREES received from TASK function: ", current_a
    print "COUNTS received from TASK function: ", current_b
    time.sleep(1)

代码说明 #

1. Lock同步：用with lock:上下文管理器自动处理锁的获取和释放，保证读写全局变量时只有一个线程在操作，避免竞态。
2. 重置逻辑转移：子线程不再主动重置a、b，而是在主线程读取完成后立即重置，确保主线程能读取到子线程设置的新值一次。
3. 阻塞处理：子线程的raw_input会阻塞，直到用户输入，避免无意义的循环。

这样修改后，就能实现你想要的效果：主线程每次读取到子线程设置的新值，输出后下一次循环就会输出0，直到子线程再次设置新值。

内容的提问来源于stack exchange，提问作者luke