Exercise posted on reddit

business_application_homework.py

# -*- coding: utf-8 -*-
"""Solution for the exercise IS340 - Business Application Programming
This exercise was posted on reddit: 
    https://www.reddit.com/r/learnprogramming/comments/fdwt9a/wrote_my_first_successful_code/
    
This exercise is to calculate the installment schedule of a car loan."""

import pandas as pd


def get_user_input():
    """Get the user input and check if it is valid.

    How it works
    ------------
    Asks the user for input, if the input is invalid, it will
    loop until a valid input is inserted.

    It asks for `purchase_price` and `loan_term` .

    Returns
    -------
    purchase_price: float
        Price without interest of the car
    loan_term: int
        Loan term in months, it can be either 24 or 36."""

    while True:
        purchase_price = input("Purchase Price: ")

        try:
            purchase_price = float(purchase_price)
            break
        except:
            print("Not a valid number")

    while True:
        loan_term_option = input("Loan Term (2 or 3 years): ")

        try:
            loan_term_option = int(loan_term_option)

            if loan_term_option == 2 or loan_term_option == 3:
                loan_term = loan_term_option * 12
                break
            else:
                raise ValueError
        except:
            print("Not a valid option")

    return purchase_price, loan_term


def calculate_scheduled_installments(purchase_price, loan_term, down_payment_rate, interest_rate):
    """Calculate the schedule table for the installments given
    `purchase_price` value and `loan_term` months.

    Arguments
    ---------
    purchase_price: float
        The price value without interest.
    loan_term: int
        Number of months for the payment.
    down_payment_rate:
        The first payment of the loan.
    interest_rate:
        Loan interest rate.

    Returns
    -------
    df: Dataframe
        A Dataframe with the data."""

    # Save the results in an array to later
    # convert into a dataframe
    results = []

    down_payment = down_payment_rate * purchase_price

    initial_balance = (purchase_price - down_payment) + \
        ((purchase_price - down_payment) * interest_rate * (loan_term / 12))

    monthly_payment = initial_balance / loan_term

    for month in range(0, loan_term):
        year = month // 12 + 1
        starting_balance = initial_balance - (month * monthly_payment)
        ending_balance = starting_balance - monthly_payment
        remaining_months = loan_term - (month + 1)

        results.append([month+1, year, starting_balance,
                        monthly_payment, ending_balance, remaining_months])

    # Create a dataframe with the results' data
    column_names = [
        'Month',
        'Year',
        'Starting Balance',
        'Installment',
        'Ending Balance',
        'Remaining Installments'
    ]

    df = pd.DataFrame(data=results, columns=column_names)

    return df


if __name__ == "__main__":

    purchase_price, loan_term = get_user_input()

    down_payment_rate = 0.1
    interest_rate = 0.06

    installment_schedule = calculate_scheduled_installments(
        purchase_price, loan_term, down_payment_rate, interest_rate)

    # Save data to excel file
    installment_schedule.to_excel('./out/busines_application_homework.xlsx')

    print(installment_schedule)