Loan calculator

calculator.py

def mortgage_calculator(form_data):
    """
    Takes a single object `form_data` and returns all calculations in a `results` object.

    Notes:
    - The `Decimal` class simply takes any number or string and casts it into a decimal
    value for calculations. See https://docs.python.org/2/library/decimal.html
    - When you see '%.2f' that's simply forcing a value to be displayed with two
    decimal places, i.e. 15.4504 comes out as 15.45.
    """
    import math
    from decimal import Decimal

    results = {}

    # Clean up POST data if required
    if 'principal' in form_data:
        form_data['principal'] = form_data['principal'].replace(',', '').replace('$', '')
    if 'annual_interest_rate' in form_data:
        form_data['annual_interest_rate'] = form_data['annual_interest_rate'].replace('%', '')
    if 'payment_amount' in form_data:
        form_data['payment_amount'] = (form_data['payment_amount'].replace(',', '').replace('$', ''))
    if 'additional_payment' in form_data:
        form_data['additional_payment'] = (form_data['additional_payment'].replace(',', '').replace('$', ''))

    # Perform calculations
    calc_type = form_data['calc_type']
    principal = form_data['principal']
    loan_years = form_data['loan_duration']
    if loan_years == 0:
        loan_years = 1
    show_amort = form_data['show_amort']
    no_payments = loan_years * 12
    air = form_data['annual_interest_rate'] * Decimal('.01')
    mir = air / 12  # E5
    pow = math.pow(mir + 1, no_payments)  # E6

    additional_payment = form_data['additional_payment']
    p_tax = form_data['property_tax']
    h_ins = form_data['hazard_insurance']
    m_ins = form_data['mortgage_insurance']
    hoa_fees = form_data['hoa_fees']
    other = form_data['other']

    payment = round(mir * (principal * Decimal(pow / (pow - 1))), 2)

    # Amortization schedule
    amort = []
    balance = principal
    payment = Decimal(payment)
    if additional_payment:
        payment = payment + additional_payment
    if p_tax:
        p_tax = Decimal(p_tax)
    else:
        p_tax = 0
    if h_ins:
        h_ins = Decimal(h_ins)
    else:
        h_ins = 0
    if m_ins:
        m_ins = Decimal(m_ins)
    else:
        m_ins = 0
    if hoa_fees:
        hoa_fees = Decimal(hoa_fees)
    else:
        hoa_fees = 0
    if other:
        other = Decimal(other)
    else:
        other = 0
    fees = Decimal(round(p_tax + h_ins + m_ins + hoa_fees + other, 2))

    # Type "I" calculations
    # Returns early and prevents the rest of the function from running
    if calc_type == 'I':
        interest_only_payment = principal * air / 12
        total_payment = interest_only_payment + fees
        results['interest_only_payment'] = '%.2f' % interest_only_payment
        results['fees'] = fees
        results['total_payment'] = '%.2f' % total_payment
        return results

    # Other calculations
    pmt_no = 0
    total_interest = 0
    total_principal = 0
    while (balance > 0):
        pmt_no += 1
        i_part = Decimal(round(balance * mir, 2))
        total_interest += i_part
        if balance < payment:
            p_part = balance
        else:
            p_part = Decimal(payment - i_part)
        total_principal += p_part
        balance = balance - p_part
        if pmt_no == no_payments:
            p_part += balance
            balance = 0
        amort.append({'pmt_no': pmt_no,
                      'i_part': '%.2f' % i_part,
                      'p_part': '%.2f' % p_part,
                      'cum_prin': '%.2f' % total_principal,
                      'cum_int': '%.2f' % total_interest,
                      'balance': '%.2f' % balance})
    results['show_amort'] = show_amort
    results['amort'] = amort
    results['fees'] = fees
    results['loan_payment'] = '%.2f' % payment
    results['total_payment'] = '%.2f' % (payment + fees)
    results['number_payments'] = pmt_no
    results['total_interest'] = round(total_interest, 2)

    return results

form.html

<form action="" class="calc-form" method="post">
    <label for="id_calc_type">Calculate...</label>
    <select id="id_calc_type" name="calc_type">
        <option value="PI">
            Principal and Interest
        </option>
        <option value="I">
            Interest Only
        </option>
    </select>

    <label for="id_principal">Loan Amount:</label>
    <input id="id_principal" name="principal" type="text">

    <label for="id_loan_duration">Loan Term (in years):</label>
    <input id="id_loan_duration" name="loan_duration" type="text">

    <label for="id_annual_interest_rate">Annual Interest Rate:</label>
    <input id="id_annual_interest_rate" name="annual_interest_rate" type="text">
   
    <label for="id_additional_payment">(Optional) Additional Monthly Principal Payment:</label>
    <input id="id_additional_payment" name="additional_payment" type="text">
    <span class="helptext">(optional) See how your amortization schedule would change by accelerating payments.</span>

    <label for="id_show_amort">Show Amortization Schedule:</label>
    <input id="id_show_amort" name="show_amort" type="checkbox">

    <label for="id_property_tax">Monthly Property Tax:</label>
    <input class="fees" id="id_property_tax" name="property_tax" type="text" value="0">
    
    <label for="id_hazard_insurance">Monthly Hazard Insurace:</label>
    <input class="fees" id="id_hazard_insurance" name="hazard_insurance" type="text" value="0">
    
    <label for="id_mortgage_insurance">Monthly Mortgage Insurance:</label>
    <input class="fees" id="id_mortgage_insurance" name="mortgage_insurance" type="text" value="0">
    
    <label for="id_hoa_fees">Monthly HOA Dues:</label>
    <input class="fees" id="id_hoa_fees" name="hoa_fees" type="text" value="0">
    
    <label for="id_other">Other:</label>
    <input class="fees" id="id_other" name="other" type="text" value="0">
</form>