Python线程执行完成后仍处于存活状态的原因排查

我在练习实现非阻塞自定义线程池（同时在了解concurrent.futures），但遇到一个问题：线程执行完成后，代码里始终显示线程处于存活状态。以下是复现代码和运行输出，请帮忙解释原因。

import threading
import time

class myThread (threading.Thread):
    def __init__(self, num):
        threading.Thread.__init__(self)
        self.num = num + 1
 
    def run(self):
        print(f">>> {self.name} is starting")
        time.sleep(self.num*2.0)
        print(f"<<< {self.name} is finishing")

num_threads = 3
threads = []
for i in range(num_threads):
    t = myThread(i)
    t.start()
    threads.append(t)

num_finished_threads = 0
while num_finished_threads < num_threads:
    time.sleep(0.5)
    for t in threads:
        if t.is_alive:
            print(f"{t.name} is alive")
        else:
            t.join()
            num_finished_threads += 1

运行输出：

>>> Thread-1 is starting
>>> Thread-2 is starting
>>> Thread-3 is starting
Thread-1 is alive
Thread-2 is alive
Thread-3 is alive
Thread-1 is alive
Thread-2 is alive
Thread-3 is alive
Thread-1 is alive
Thread-2 is alive
Thread-3 is alive
<<< Thread-1 is finishing
Thread-1 is alive
Thread-2 is alive
Thread-3 is alive
Thread-1 is alive
Thread-2 is alive
Thread-3 is alive
Thread-1 is alive
Thread-2 is alive
Thread-3 is alive
Thread-1 is alive
Thread-2 is alive
Thread-3 is alive
<<< Thread-2 is finishing
Thread-1 is alive
Thread-2 is alive
Thread-3 is alive
Thread-1 is alive
Thread-2 is alive
Thread-3 is alive
Thread-1 is alive
Thread-2 is alive
Thread-3 is alive
Thread-1 is alive
Thread-2 is alive
Thread-3 is alive
<<< Thread-3 is finishing
Thread-1 is alive
Thread-2 is alive
Thread-3 is alive

核心错误是你把线程的is_alive方法当成了属性来使用：

• t.is_alive是一个方法对象，只要这个方法存在，条件判断就会返回True，完全不考虑线程实际的存活状态。
• 正确的用法应该是调用这个方法：t.is_alive()，它会返回布尔值来表示线程当前是否存活。

因为这个错误，你的代码永远不会进入else分支，num_finished_threads永远不会递增，循环会一直执行；同时线程结束后也不会被调用join()，但本质上线程已经结束了，只是你的判断逻辑错误导致误以为它还处于存活状态。

import threading
import time

class myThread (threading.Thread):
    def __init__(self, num):
        threading.Thread.__init__(self)
        self.num = num + 1
 
    def run(self):
        print(f">>> {self.name} is starting")
        time.sleep(self.num*2.0)
        print(f"<<< {self.name} is finishing")

num_threads = 3
threads = []
for i in range(num_threads):
    t = myThread(i)
    t.start()
    threads.append(t)

num_finished_threads = 0
while num_finished_threads < num_threads:
    time.sleep(0.5)
    for t in threads:
        if t.is_alive():  # 改为调用is_alive方法
            print(f"{t.name} is alive")
        else:
            t.join()
            num_finished_threads += 1

内容的提问来源于stack exchange，提问作者DuGNu