a.py

import threading

x = 0

def increament():
    global x
    x +=1

def thread_task(lock):
    for _ in range(100000):
        lock.acquire()
        increament()
        lock.release()

def main_task():
    global x
    x = 0
    lock = threading.Lock()
    t1 = threading.Thread(target=thread_task, args=(lock, ))
    t2 = threading.Thread(target=thread_task, args=(lock, ))
    t3 = threading.Thread(target=thread_task, args=(lock, ))

    t1.start()
    t2.start()
    t3.start()

    t1.join()
    t2.join()
    t3.join()

if __name__=='__main__':
    for i in range(10):
        main_task()
        print('Iterator {}: x = {}'.format(i, x))