用 Python 计算抵押贷款利息

Posted 2023-02-23

【中文标题】用 Python 计算抵押贷款利息

【英文标题】：Calculating mortgage interest in Python

【发布时间】：2015-06-30 12:58:27

【问题描述】：

我目前正在通过 youtube 上的视频教程学习 python，并且遇到了一个我似乎无法掌握的公式，因为我觉得没有什么是正确的。练习的基本概念是制作一个抵押计算器，要求用户输入 3 条信息，贷款金额、利率和贷款期限（年）

然后它计算每月支付给用户的费用。这是我的代码：

__author__ = 'Rick'
# This program calculates monthly repayments on an interest rate loan/mortgage.

loanAmount = input("How much do you want to borrow? \n")
interestRate = input("What is the interest rate on your loan? \n")
repaymentLength = input("How many years to repay your loan? \n")

#converting the string input variables to float
loanAmount = float(loanAmount)
interestRate = float(interestRate)
repaymentLength = float(repaymentLength)

#working out the interest rate to a decimal number
interestCalculation = interestRate / 100

print(interestRate)
print(interestCalculation)

#working out the number of payments over the course of the loan period.
numberOfPayments = repaymentLength*12

#Formula
#M = L[i(1+i)n] / [(1+i)n-1]

#   * M = Monthly Payment (what were trying to find out)
#   * L = Loan Amount (loanAmount)
#   * I = Interest Rate (for an interest rate of 5%, i = 0.05 (interestCalculation)
#   * N = Number of Payments (repaymentLength)

monthlyRepaymentCost = loanAmount * interestCalculation * (1+interestCalculation) * numberOfPayments / ((1+interestCalculation) * numberOfPayments - 1)
#THIS IS FROM ANOTHER BIT OF CODE THAT IS SUPPOSE TO BE RIGHT BUT ISNT---
# repaymentCost = loanAmount * interestRate * (1+ interestRate) * numberOfPayments  / ((1 + interestRate) * numberOfPayments -1)

#working out the total cost of the repayment over the full term of the loan
totalCharge = (monthlyRepaymentCost * numberOfPayments) - loanAmount


print("You want to borrow £" + str(loanAmount) + " over " + str(repaymentLength) + " years, with an interest rate of " + str(interestRate) + "%!")

print("Your monthly repayment will be £" + str(monthlyRepaymentCost))

print("Your monthly repayment will be £%.2f " % monthlyRepaymentCost)

print("The total charge on this loan will be £%.2f !" % totalCharge)

一切正常，但它最终抛出的价值是完全错误的...... 100 英镑的贷款，利率为 10% 超过 1 年不应该让我每月支付 0.83 英镑。任何帮助我理解这个等式以帮助我理解的帮助将不胜感激。

【问题讨论】：

某处您只是支付利息而不是增加贷款金额... 0.83 x 12 = ~10

利息多久累积一次？每个月？

我不确定，这是我在模块之后得到的指令。创建一个抵押/贷款计算器。 * 让用户输入贷款成本、利率和贷款年数 * 使用以下公式计算每月还款额 * * M = L[i(1+i)n] / [(1 +i)n-1] * M = 每月还款 * L = 贷款金额 * I = 利率（对于 5% 的利率，i = 0.05 * N = 还款次数

这个程序不计算复利或单利。它也没有提供复利期，它也错误地显示了您的总费用，不要减去贷款金额。它也只是将您的利率除以您的付款次数以获得每月付款金额，并且实际上并未使用复利或单利的公式。你真的没有正确实现注释掉的公式，你可能想再看看。

这取决于抵押贷款供应商的小字，他们准确地说出“年利率”的含义:-) 处理它的一种粗略（但不是真正正确）的方法是将年利率除以利率提高 12 倍。使用monthly_rate = (1 + annual_rate)**(1/12.) - 1 之类的东西会更好。

【参考方案1】：

显然你把公式复制错了。

wrong:   * numberOfPayments 

correct: ** numberOfPayments 

注意：它在公式中出现两次 注意：在 python 中，** 是“幂”运算符。

【参考方案2】：

我也遇到了这个问题，这是我的解决方案

loan = input('Enter Loan Amount: ')
loan = float(loan)

numberOfPayments = input('Enter Loan payments in years: ')
numberOfPayments = float(numberOfPayments) * 12

interest = input('Annuel interest Rate: ')
interest = float(interest)/100/12

monthlyPayments = loan * (interest * (1 + interest) ** numberOfPayments) / ((1 + interest) ** numberOfPayments - 1)

print (round(monthlyPayments))

【参考方案3】：

我也观看了这个 youtube 视频并且遇到了同样的问题。 我是 python 新手，所以请耐心等待。 视频中的讲师使用的是python3，我使用的是python2 所以我不确定所有语法是否都相同。但是使用此线程中的一些信息和此链接中的信息：(http://www.wikihow.com/Calculate-Mortgage-Payments) 我能够得到答案。 （我的计算结果与在线抵押贷款计算器相符）

#!/usr/bin/env python
# The formula I used:
M = L * ((I * ((1+I) ** n)) / ((1+I) ** n - 1))

# My code (probably not very eloquent but it worked)

# monthly payment
M = None
# loan_amount
L = None
# interest rate
I = None
# number of payments
n = None

L = raw_input("Enter loan amount: ")
L = float(L)
print(L)

I = raw_input("Enter interest rate: ")
I = float(I)/100/12
print(I)

n = raw_input("Enter number of payments: ")
n = float(n)
print(n)

M = L * ((I * ((1+I) ** n)) / ((1+I) ** n - 1))

M = str(M)
print("\n")
print("Monthly payment is " + M)

【讨论】：

你为什么使用 Python 2 而不是 Python 3？如果您不必支持旧代码库，似乎使用 Python 3 会更好。

将利率除以 12 可能是错误的，具体取决于用户输入的是 APR 还是 APY。 reference.com/business-finance/…

【参考方案4】：

借助示例，这就是我所做的。

# Formula for mortgage calculator
# M = L(I(1 + I)**N) / ((1 + I)**N - 1)
# M = Monthly Payment, L = Loan, I = Interest, N = Number of payments, ** = exponent

# Declares and asks for user to input loan amount. Then converts to float
loanAmount = input('Enter loan amount \n')
loanAmount = float(loanAmount)

# Declares and asks user to input number of payments in years. Then converts to float. Years * 12 to get
#  total number of months
years = input('How many years will you have the loan? \n')
years = float(years) * 12

# Declares and asks user to input interest rate. Then converts to float and input interest rate is /100/12
interestRate = input('Enter Interest Rate \n')
interestRate = float(interestRate) / 100 / 12

# Formula to calculate monthly payments
mortgagePayment = loanAmount * (interestRate * (1 + interestRate)
                                ** years) / ((1 + interestRate) ** years - 1)

# Prints monthly payment on next line and reformat the string to a float using 2 decimal places
print("The monthly mortgage payment is\n (%.2f) " % mortgagePayment)

【参考方案5】：

这对我来说效果很好。不准确，因为贷款计算器通常会按天计算，而我很懒所以会按月计算，但这是我写的一个简单的计算器，它足够准确实用。

L = input("How much will you be borrowing? ")
L = float(L)
print(L)


N = input("How many years will you be paying this loan off? ")
N = float(N) *12
print(N)

I = input("What is the interest in percents that you will be paying? Ex, 10% = 10, 5% = 5, etc. ")
I = float(I)/100
print(I)

M = (L/N) + I*(L/N)

float(M) 
print("Your monthly payment will be: ")
print(M)

【参考方案6】：

这个mortgage 包做得很好：

>>> import mortgage
>>> m=mortgage.Mortgage(interest=0.0375, amount=350000, months=360)
>>> mortgage.print_summary(m)
                     Rate:      0.037500
             Month Growth:      1.003125
                      APY:      0.038151
             Payoff Years:            30
            Payoff Months:           360
                   Amount:     350000.00
          Monthly Payment:       1620.91
           Annual Payment:      19450.92
             Total Payout:     583527.60