Command Line Loan Calculator

I wrote this program as part of the Python Developer Path by JetBrains Academy (https://hyperskill.org). It is operated from the command line and allows you to calculate monthly payments, the principal amount, or the number of months it takes to repay a loan.

There are two different types of payment: differentiated and annuity payment.

The program requires four parameters.

Possible parameters are:

--type (always required), indicating the type of payment ("annuity" or "diff")
--payment, specifying the monthly payment. This can only be provided for annuity payment, since the montly payments differ in differentiated payment
--principal, referring to the loan principal
--periods, referring to the number of months needed to repay the loan
--interest (always required), denoting the interest rate. It is specified withour the %-sign

The calculator is able to calculate either the monthly payment, or the loan principal, or the periods, depending on which parameter you do not provide when starting it.

Examples

Calculating differentiated payments for a 10-month loan with a principal of 1,000000 at 10% interest

$ python creditcalc.py --type=diff --principal=1000000 --periods=10 --interest=10

Month 1: paid out 108334
Month 2: paid out 107500
Month 3: paid out 106667
Month 4: paid out 105834
Month 5: paid out 105000
Month 6: paid out 104167
Month 7: paid out 103334
Month 8: paid out 102500
Month 9: paid out 101667
Month 10: paid out 100834
Overpayment = 45837

Calculating the annuity payment for a 120-month (10-year) loan with a principal amount of 1,000,000 at 12% interest

$ python creditcalc.py --type=annuity --principal=1000000 --periods=120 --interest=12

Your annuity payment = 14348!
Overpayment = 721760

Calculating differentiated payments given a principal of 700,000 over 6 months at an interest rate of 9.7%

$ python creditcalc.py --type=diff --principal=700000 --periods=6 --interest=9.7

Month 1: paid out 122325
Month 2: paid out 121382
Month 3: paid out 120439
Month 4: paid out 119496
Month 5: paid out 118553
Month 6: paid out 117610
Overpayment = 19805

Calculating the principal for a user paying 5,050 per month for 120 months (10 years) at 5.6% interest