Simple PMI calculator that accepts the loan amount, interest and term in years.

pmi.py

#!/usr/bin/env python2.7
'''
Simple PMI calculator for monthly payments.

Enter the loan amount, the annual interest rate and the number
of years.
'''
import sys


def pmi(amt, interest, term, ppyr=12):
    '''
    Create an amortization schedule based on the term
    of the loan.

    @param amt       The total amount of the loan.
    @param interest  The interest rate.
    @param term      Term of the loan.
    '''
    # CITATION: https://www.mtgprofessor.com/formulas.htm
    # CITATION: https://en.wikipedia.org/wiki/Amortization_calculator
    # pylint: disable=too-many-locals
    famt = float(amt)
    fint = float(interest)
    term = int(term)

    pir = fint / float(ppyr)  # periodic interest rate
    pay = famt * (pir + (pir / (((1. + pir) ** term) - 1.)))
    total_paid = pay * term
    interest_paid = total_paid - famt
    fmt = '   {:>2}  {:>4}  {:>12}  {:>12}  {:>12}  {:>12}  {:>12}'

    def currency(val):
        '''
        Convert to currency.
        '''
        return '${:,.2f}'.format(val)

    # pylint: disable=superfluous-parens
    print('')
    print('Amount:   {:>12}'.format(currency(famt)))
    print('Interest: {:12,.2f}  {:.2f}%'.format(fint, 100. * fint))
    print('Term:     {:12,}'.format(term))

    print('PerInt:   {:12,.4f}  {:.2f}%'.format(pir, 100. * pir))
    print('Payment:  {:>12}'.format(currency(pay)))
    print('Total:    {:>12}'.format(currency(total_paid)))
    print('TotalInt: {:>12}'.format(currency(interest_paid)))
    print('')
    print(fmt.format('Yr',
                     'Per',
                     'Principal',
                     'Payment',
                     'PrincPaid',
                     'IntPaid',
                     'AccTotal'))
    # pylint: enable=superfluous-parens

    prev_total_paid_off = 0
    acc_payments = pay
    for i in range(1, term+1):
        num = ((1. + pir) ** i) - 1
        div = ((1. + pir) ** term) - 1
        pri = famt * (1. - (num / div))  # principal remaining
        total_paid_off = famt - pri
        paid_off_this_period = total_paid_off - prev_total_paid_off
        interest_paid = pay - paid_off_this_period
        # pylint: disable=superfluous-parens
        print(fmt.format(((i+11)//12),
                         i,
                         currency(pri),
                         currency(pay),
                         currency(paid_off_this_period),
                         currency(interest_paid),
                         currency(acc_payments)))
        # pylint: enable=superfluous-parens
        prev_total_paid_off = total_paid_off
        acc_payments += pay
    # pylint: enable=too-many-locals


def main():
    '''
    Main
    '''
    def get_val(prompt, i):
        '''
        Get a value.
        '''
        if len(sys.argv) > i:
            return sys.argv[i]
        val = raw_input(prompt)
        try:
            float(val)
        except ValueError:
            sys.stderr.write('ERROR: Not a valid number for "{}".\n'.format(prompt))
            sys.exit(1)
    amount = get_val('Loan amount? ', 1)
    interest = get_val('Interest rate? ', 2)
    term = int(get_val('Number of years? ', 3))
    pmi(amount, interest, term*12)


if __name__ == '__main__':
    main()

Updated to work with python-3.