Python multithreading bug

August 30, 2024

I am new to python, and I created a Python script that employs multithreading to increase a shared counter. I anticipated that the final value of the counter would reach 1,000,000, as I am executing 10 threads, each adding to the counter 100,000 times. Nevertheless, the outcome consistently falls short of 1,000,000.

Here’s the code I used:

import threading

counter = 0

def increment_counter():
    global counter
    for _ in range(100000):
        counter += 1

threads = []
for _ in range(10):
    thread = threading.Thread(target=increment_counter)
    threads.append(thread)
    thread.start()

for thread in threads:
    thread.join()

print("Final counter value:", counter)

>Solution :

The problem is from a race condition due to the absence of synchronization when multiple threads attempt to modify the counter at the same time. To resolve this issue, implement a threading.Lock to ensure that only one thread can increase the counter simultaneously:

import threading

counter = 0
lock = threading.Lock()

def increment_counter():
    global counter
    for _ in range(100000):
        with lock:
            counter += 1

threads = []
for _ in range(10):
    thread = threading.Thread(target=increment_counter)
    threads.append(thread)
    thread.start()

for thread in threads:
    thread.join()

print("Final counter value:", counter)