شمارندهٔ امن در چهار ریسه

import threading

counter = 0
lock = threading.Lock()

def worker(n):
    global counter
    for _ in range(n):
        with lock:
            counter += 1

threads = [threading.Thread(target=worker, args=(10_000,)) for _ in range(4)]
for t in threads: t.start()
for t in threads: t.join()
print(counter)  # 40000