变量在while循环中不断更改回其原始值

Question

我在做MITx 6.00.01x课程，我在第三道题的第二道题上，我卡住了。 我的代码：

    balance = 320000
    annualInterestRate = 0.2
    monthlyInterestRate = (annualInterestRate) / 12.0
    monthlyFixedPayment = 0
    empBalance = balance
    lowerBound = round((balance)/12,2)
    upperBound = (balance*(1+monthlyInterestRate)**12)/12
    monthlyFixedPayment = round( ( (lowerBound+upperBound)/2) ,2)
    while tempBalance != 0: 
        monthlyFixedPayment = round( ( (lowerBound+upperBound)/2) ,2)  
        for m in range(12) :
            tempBalance -= monthlyFixedPayment 
            tempBalance += (monthlyInterestRate)*(tempBalance)
            tempBalance = round(tempBalance,2) 
        if tempBalance > 0:
            lowerBound = round(monthlyFixedPayment,2)
            tempBalance = balance
        elif tempBalance < 0: 
            upperBound = round(monthlyFixedPayment,2)
            tempBalance = balance

    print('Lowest Payment: ' + str(round(monthlyFixedPayment,2)))

我的代码使用二分法搜索来生成monthlyFixedPayment，但是当我到达末尾的行，该行更改了上界值或下界值，然后再次启动循环后，下界值和上界值将重置为循环外的值。有人知道怎么防止吗？你知道吗