A simple Mortgage Calculator

''' mortgage_loan_calc1.py
calculate the monthly payment on a mortgage loan
tested with Python27 and Python33
'''

import math

def calc_mortgage(principal, interest, years):
    '''
    given mortgage loan principal, interest(%) and years to pay
    calculate and return monthly payment amount
    '''
    # monthly rate from annual percentage rate
    interest_rate = interest/(100 * 12)
    # total number of payments
    payment_num = years * 12
    # calculate monthly payment
    payment = principal * \
        (interest_rate/(1-math.pow((1+interest_rate), (-payment_num))))
    return payment


# mortgage loan principal
principal = 100000
# percent annual interest
interest = 7.5
# years to pay off mortgage
years = 30
# calculate monthly payment amount
monthly_payment = calc_mortgage(principal, interest, years)
# calculate total amount paid
total_amount = monthly_payment * years * 12

# show result ...
# {:,} uses the comma as a thousands separator
sf = '''\
For a {} year mortgage loan of ${:,}
at an annual interest rate of {:.2f}%
you pay ${:.2f} monthly'''
print(sf.format(years, principal, interest, monthly_payment))
print('-'*40)
print("Total amount paid will be ${:,.2f}".format(total_amount))

''' result ...
For a 30 year mortgage loan of $100,000
at an annual interest rate of 7.50%
you pay $699.21 monthly
----------------------------------------
Total amount paid will be $251,717.22
'''