problem : python threads still alive after completion

Question

As an exercise, I try to implement a non blocking mechanism to manage my own pool of threads (I know concurrent.futures). I ended with a problem: threads are always alive, even after they have completed.

I have written this little code to expose the problem.

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

Can anyone explain me the behaviour of python: why those threads are still alive after completion ?

>>> 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

Thanks a lot.

Solomon Slow · Answer 1 · 2022-12-15T20:07:49.510

Try calling the is_alive() function instead of testing the is_alive attribute.

    if t.is_alive():
        print(f"{t.name} is alive")

The reason why you didn't get an "...object has no attribute..." error when you tested t.is_alive is because a Thread instance has an attribute with that name. But the value of that attribute is the function, and a function is always "truthy."

In order to find out whether or not the thread is alive, you have to call the function.

https://docs.python.org/3/library/threading.html#threading.Thread.is_alive

score 0 · Answer 2 · answered Dec 20 '22 at 09:28

You're thread will remain alive unless you join it.
Create a separate for loop to join all threads.
This will wait for the execution to complete, and will close the threads

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

    for t in threads:
        t.join()
        num_finished_threads += 1
print("done")



Thread-12 is starting>>> Thread-13 is starting

Thread-14 is starting
Thread-12 is alive
Thread-13 is alive
Thread-14 is alive
Thread-12 is finishing
Thread-13 is finishing
Thread-14 is finishing
done

problem : python threads still alive after completion

2 Answers2