为何break命令无法让多线程数字查找程序停止运行？

问题描述 #

你编写了三个线程分别在不同数字区间内查找输入的数字，当某个线程找到目标数字并执行break后，程序并未停止，而是继续运行其他线程的循环。代码如下：

import threading
import time    

def thread_func_1():
    
    for i in range(1000,3334):
        time.sleep(0.02)
        print(f"thred1 {i}")
        if number == i:
            print(f"1.. Thread şifreyi buldu: {i}")
            break
            

def thread_func_2():
    
    for i in range(3334,6667):
        time.sleep(0.02)
        print(f"thred2 {i}")
        
        if number == i:
            print(f"2. Thread şifreyi buldu: {i}")
            break

def thread_func_3():
    
    for i in range(6667,10000):
        time.sleep(0.02)
        print(f"thred3 {i}")
        
        if number == i:
            print(f"3. Thread şifreyi buldu: {i}")
            break


print("Lütfen bir sayı giriniz: ")
number = int(input())

t1 = threading.Thread(target=thread_func_1)
t2 = threading.Thread(target=thread_func_2)
t3 = threading.Thread(target=thread_func_3)

t1.start()
t2.start()
t3.start()

原因分析 #

• break语句只能终止当前线程内的for循环，让执行该语句的线程结束，但无法影响其他正在运行的线程。
• Python程序默认会等待所有非守护线程执行完毕后才会退出。你启动的三个线程都是非守护线程，所以即使其中一个线程结束，另外两个线程仍会继续执行循环，导致程序无法停止。

解决方法 #

方法1：使用全局标志位控制线程退出 #

定义一个全局变量作为停止标志，每个线程在循环时检查该标志，一旦标志为True就终止循环：

import threading
import time

# 全局停止标志
stop_flag = False

def thread_func_1():
    global stop_flag
    for i in range(1000,3334):
        if stop_flag:
            break
        time.sleep(0.02)
        print(f"thred1 {i}")
        if number == i:
            print(f"1.. Thread şifreyi buldu: {i}")
            stop_flag = True
            break
            

def thread_func_2():
    global stop_flag
    for i in range(3334,6667):
        if stop_flag:
            break
        time.sleep(0.02)
        print(f"thred2 {i}")
        if number == i:
            print(f"2. Thread şifreyi buldu: {i}")
            stop_flag = True
            break

def thread_func_3():
    global stop_flag
    for i in range(6667,10000):
        if stop_flag:
            break
        time.sleep(0.02)
        print(f"thred3 {i}")
        if number == i:
            print(f"3. Thread şifreyi buldu: {i}")
            stop_flag = True
            break


print("Lütfen bir sayı giriniz: ")
number = int(input())

t1 = threading.Thread(target=thread_func_1)
t2 = threading.Thread(target=thread_func_2)
t3 = threading.Thread(target=thread_func_3)

t1.start()
t2.start()
t3.start()

方法2：使用threading.Event实现线程间通信 #

Event是线程安全的通信机制，比全局变量更可靠：

import threading
import time

# 创建事件对象
stop_event = threading.Event()

def thread_func_1():
    for i in range(1000,3334):
        if stop_event.is_set():
            break
        time.sleep(0.02)
        print(f"thred1 {i}")
        if number == i:
            print(f"1.. Thread şifreyi buldu: {i}")
            stop_event.set()
            break
            

def thread_func_2():
    for i in range(3334,6667):
        if stop_event.is_set():
            break
        time.sleep(0.02)
        print(f"thred2 {i}")
        if number == i:
            print(f"2. Thread şifreyi buldu: {i}")
            stop_event.set()
            break

def thread_func_3():
    for i in range(6667,10000):
        if stop_event.is_set():
            break
        time.sleep(0.02)
        print(f"thred3 {i}")
        if number == i:
            print(f"3. Thread şifreyi buldu: {i}")
            stop_event.set()
            break


print("Lütfen bir sayı giriniz: ")
number = int(input())

t1 = threading.Thread(target=thread_func_1)
t2 = threading.Thread(target=thread_func_2)
t3 = threading.Thread(target=thread_func_3)

t1.start()
t2.start()
t3.start()

方法3：设置守护线程（不推荐） #

把线程设为守护线程，这样主线程结束时会强制终止所有守护线程。但这种方式可能导致线程未完成清理工作就被终止，仅适用于不需要优雅退出的场景：

# ... 前面函数代码不变 ...

t1 = threading.Thread(target=thread_func_1, daemon=True)
t2 = threading.Thread(target=thread_func_2, daemon=True)
t3 = threading.Thread(target=thread_func_3, daemon=True)

t1.start()
t2.start()
t3.start()

# 主线程等待任意一个线程结束后退出
t1.join()
t2.join()
t3.join()

内容的提问来源于stack exchange，提问作者Fatih