好像不加这个初始值就运行不了

相关代码：

​import threading

balance = 0

def change_it(n):
    """改变我的余额"""
    global balance
    balance = balance + n
    balance = balance - n
    print('----------------{0}'.format(n))

class ChangeBalanceThread(threading.Thread):

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


    def run(self):
        for i in range(1000):
            change_it(self.num)



if __name__ == '__main__':
    t1 = ChangeBalanceThread(5)
    t2 = ChangeBalanceThread(8)
    t1.start()
    t2.start()
    t1.join()
    t2.join()
    print('the last:{0}'.format(balance))