Python Forum

I have a factory method that may be called from many threads. The method looks inside the thread locals for a certain object and if it doesn't exist, creates one and stores it in thread locals and then returns the same.

The object created thus has a __del__ method implemented for cleanup.

Here is a small code that explains what I am doing . My expectation is that, there are only two objects created by the factory one in main thread and other in the daemon thread.

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
import time
import threading


class MysteryObject:
    def __init__(self, name):
        self.name = name

    def __del__(self):
        print('{} => {} destroyed'.format(self.__class__.__name__, self.name))


def get_thread_ident():
    current = threading.currentThread()
    return '_'.join([current.getName(), str(current.ident)])


def make_object():
    thread_ident = get_thread_ident()
    thread_locals = threading.local()
    if hasattr(thread_locals, 'mystery_object'):
        print('found mystery object in thread {}'.format(thread_ident))
        return thread_locals.mystery_object
    print('Creating mystery object in thread {}'.format(thread_ident))
    thread_locals.mystery_object = MysteryObject(thread_ident)
    return thread_locals.mystery_object


def test_threaded_object_maker(total_iteration):
    iteration = 0
    my_obj = make_object()
    while iteration < total_iteration:
        my_cur_obj = make_object()
        if id(my_cur_obj) != id(my_obj):
            print('{} Thread mystery object was GCed'.format(my_obj.name))
            my_obj = my_cur_obj
        time.sleep(2)
        iteration += 2


def test_object_maker_main():
    main_thread_mystery = make_object()
    threads = []
    for _ in range(1):
        threads.append(threading.Thread(target=test_threaded_object_maker, args=(5,)))
        threads[-1].start()
    for th in threads:
        th.join()
    main_thread_mystery2 = make_object()
    if id(main_thread_mystery) != id(main_thread_mystery2):
        print('Error main thread mystery object was GCed')


if __name__ == '__main__':
    test_object_maker_main()

But when I run the code I see ...

Output:
Creating mystery object in thread MainThread_140259104548672
Creating mystery object in thread Thread-1_140259096856320
Creating mystery object in thread Thread-1_140259096856320
Thread-1_140259096856320 Thread mystery object was GCed
MysteryObject => Thread-1_140259096856320 destroyed
Creating mystery object in thread Thread-1_140259096856320
Thread-1_140259096856320 Thread mystery object was GCed
MysteryObject => Thread-1_140259096856320 destroyed
Creating mystery object in thread Thread-1_140259096856320
Thread-1_140259096856320 Thread mystery object was GCed
MysteryObject => Thread-1_140259096856320 destroyed
MysteryObject => Thread-1_140259096856320 destroyed
Creating mystery object in thread MainThread_140259104548672
Error main thread mystery object was GCed
MysteryObject => MainThread_140259104548672 destroyed
MysteryObject => MainThread_140259104548672 destroyed

I expect objects in thread locals to stick around until the thread ends. What am I missing?
I also created a stackoverflow post but haven't got any answers yet.