问下这段代码错误在哪里？

import threading
import time


balance = 100

lock = threading.Lock()
lock2 = threading.Lock()

def add(n):
   lock2.acquire()
   try:
       global balance
       balance += n
       print(balance)
   finally:
       lock2.release()
       #print(balance)
       return balance


def mins(n):
   lock.acquire()
   try:
       global balance
       balance -= n
   finally:
       lock.release()
       print(balance)
       return balance


'''
def change(n):
   add(n)
   mins(n)
   print('n: {}, balance:{}'.format(n,balance))

'''

class threadTest(threading.Thread):

   lock = threading.Lock()
   def __init__(self,number,*args,**kwargs):
       super().__init__(*args,**kwargs)
       self.number = number

   def run(self):
       i = 0
       for i in range(10):
           self.lock.acquire()
           try:
               n = add(self.number)
               print('add({}):   balance:{}'.format(i,n))
               i += 1
           finally:
               self.lock.release()



class threadTest2(threading.Thread):

   lock = threading.Lock()

   def __init__(self,number,*args,**kwargs):
       super().__init__(*args,**kwargs)
       self.number = number


   def run(self):
       i = 0
       for i in range(10):
           self.lock.acquire()
           try:
               n = mins(self.number)
               print('mins({}):  balance:{}'.format(i,n))
               i += 1
           finally:
               self.lock.release()






if __name__ == '__main__':

   t = threadTest(1)
   t3 = threadTest2(5)
   t1 = threadTest(5)
   t2 = threadTest2(1)

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

'''
   i = 0
   while i < 10:
        add(10)
        mins(10)
        i += 1
'''

老师问下这段代码的错误在哪里。。我不是已经锁住了么,每次运行到最后打印出来的结果也是balance不变，但是程序内储存的结果balance每次都会变化，这是为什么呢？