
import threading 
  
# global variable x 
x = 0
  
def increment(): 
    """ 
    function to increment global variable x 
    """
    global x 
    x += 1
  
def thread_task(lock): 
    """ 
    task for thread 
    calls increment function 100000 times. 
    """
    for _ in range(100000): 
        lock.acquire() # ロック　※１
        increment() 
        lock.release() # ロック解除　※１
  
def main_task(): 
    global x 
    # setting global variable x as 0 
    x = 0
  
    # creating a lock 
    lock = threading.Lock() 
  
    # creating threads 
    t1 = threading.Thread(target=thread_task, args=(lock,)) 
    t2 = threading.Thread(target=thread_task, args=(lock,)) 
  
    # start threads 
    t1.start() 
    t2.start() 
  
    # t1, t2 を順番処理させる 
    t1.join() 　#※２
    t2.join() 　#※２
  
if __name__ == "__main__": 
    for i in range(10): 
        main_task() 
        print("Iteration {0}: x = {1}".format(i,x))

正しい結果

Iteration 0: x = 200000
Iteration 1: x = 200000
Iteration 2: x = 200000
Iteration 3: x = 200000
Iteration 4: x = 200000
Iteration 5: x = 200000
Iteration 6: x = 200000
Iteration 7: x = 200000
Iteration 8: x = 200000
Iteration 9: x = 200000

ちなみに
※１の部分だけをコメントアウトすると

Iteration 0: x = 184418
Iteration 1: x = 177826
Iteration 2: x = 200000
Iteration 3: x = 150490
Iteration 4: x = 179985
Iteration 5: x = 200000
Iteration 6: x = 176347
Iteration 7: x = 165817
Iteration 8: x = 176203
Iteration 9: x = 132321

※２の部分だけをコメントアウトすると

Iteration 0: x = 23071
Iteration 1: x = 18026
Iteration 2: x = 32923
Iteration 3: x = 7482
Iteration 4: x = 102353
Iteration 5: x = 65894
Iteration 6: x = 65155
Iteration 7: x = 52110
Iteration 8: x = 41988
Iteration 9: x = 52410