amort/amort.py

import argparse, json

def generate_amortization_schedule(principal, interest_rate, loan_term, one_time_payment=None):
    # Convert interest rate to decimal and calculate periodic interest rate
    monthly_interest_rate = interest_rate / 12 / 100

    # Calculate the number of monthly payments
    num_payments = loan_term * 12

    # Calculate the fixed monthly payment using the amortization formula
    monthly_payment = (principal * monthly_interest_rate) / (1 - (1 + monthly_interest_rate) ** -num_payments)
    monthly_payment = round(monthly_payment, 2)

    # Initialize variables
    remaining_balance = principal
    amortization_schedule = []

    for payment_number in range(1, num_payments + 1):
        # Calculate interest for the current period
        interest_payment = round(remaining_balance * monthly_interest_rate, 2)

        # Calculate principal payment
        principal_payment = round(monthly_payment - interest_payment, 2)

        # Apply one-time payment if provided
        if one_time_payment and payment_number == one_time_payment['payment_number']:
            principal_payment += one_time_payment['amount']
            remaining_balance -= one_time_payment['amount']

        # Update remaining balance
        remaining_balance -= principal_payment

        if remaining_balance < 0:
            principal_payment = principal_payment + remaining_balance
            monthly_payment = monthly_payment + remaining_balance
            remaining_balance = 0

        # Create a dictionary with payment details and add it to the amortization schedule
        payment_details = {
            'Payment Number': payment_number,
            'Payment Amount': round(monthly_payment,2),
            'Interest Payment': interest_payment,
            'Principal Payment': round(principal_payment,2),
            'Remaining Balance': round(remaining_balance,2)
        }
        amortization_schedule.append(payment_details)

    return amortization_schedule

if __name__ == "__main__":
    # Example usage
    # principal = 100000
    # interest_rate = 5.0
    # loan_term = 3
    def get_arguments():
        p = argparse.ArgumentParser()
        p. add_argument("--principal", "-p", type=float, \
            help="set value for principal")
        p.add_argument("---interest-rate", "-i", type=float,\
            help="set the value for interest rate (percentage)")
        p.add_argument("--term", "-t", type=int,\
            help="sets the term (years)")
        p.add_argument("--one-time", "-ot", type=str,\
            help="factors in a one-time payment (json, example: {\"payment_number\":13,\"amount\":5000}")
        args = p.parse_args()
        return args.principal, args.interest_rate, args.term, args.one_time
    
    principal, interest_rate, loan_term, one_time = get_arguments()
    if one_time is not None:
        one_time_payment = json.loads(one_time)
    else: one_time_payment = None
    # one_time_payment = {
    #     'payment_number': 13,
    #     'amount': 5000
    # }

    schedule = generate_amortization_schedule(principal, interest_rate, loan_term, one_time_payment)

    total_paid = 0
    total_interest_paid = 0
    total_principal_paid = 0

    # Display the amortization schedule
    print("id, paid, interest payment, principal payment, remaining")
    for payment in schedule:
        # print(payment)
        total_paid += payment["Payment Amount"]
        total_interest_paid += payment["Interest Payment"]
        total_principal_paid += payment["Principal Payment"]  
        if payment["Remaining Balance"] < 0:
            break
        attrs = [payment[key] for key in payment]
        print("%s" % ", ".join([str(attr) for attr in attrs]))

    print("total paid: ", round(total_paid,2))
    print("total interest paid: ", round(total_interest_paid,2))
    print("total principal paid: ", round(total_principal_paid,2))