mortgage/mortgage.py

import json
from decimal import *
from datetime import *

# this program reads the json file describing a mortgage and calculate the actual and projected amortization
# the first payment in the file should be the interest paid at closing
# the interest at closing covers interest that would be incurred from the date of closing until the first day
# that will be covered by the first loan payment
# i.e., the first loan payment is expected a month and a half after closing, the first two weeks interest is due
# at closing.  The first payment will incur interest from one month before the bill is due.

#read in the file
#filename = "./10Kloan.txt"
filename = "./dadmortgage.txt"
#filename = "./brendamortgage.txt"
#filename = "./greenfield_mortgage.txt"

if filename:
    with open(filename, 'r') as f:
        datastore = json.load(f)

#read in the loan profile information
annual_rate = Decimal(datastore['interest rate'])/100
daily_interest_rate = annual_rate/360
principal = Decimal(datastore["principal"]).quantize(Decimal("1.00"))
periods_per_year = Decimal(datastore["periods per year"])
total_periods = Decimal(datastore["periods"])
payment_day_of_month = int(datastore['payment day of month'])

if "monthly_payment" in datastore:
    monthly_payment = Decimal(datastore["monthly_payment"]).quantize(Decimal("1.00"))
else:
    #calculate expected monthly payment
    periodic_rate = annual_rate/periods_per_year
    discount_factor = (((1+periodic_rate)**total_periods)-1) / (periodic_rate * ((1+periodic_rate)**total_periods))
    monthly_payment = (principal / discount_factor).quantize(Decimal("1.00"))



print "Principal: ", principal
print "Interest Rate: ", annual_rate
print "Payments Per Year: ", periods_per_year
print "Loan Term: ", total_periods
print "Monthly Payment", monthly_payment
print "Payments due day: ", payment_day_of_month

print

#loop over the payments and calculate the actual amortization
actual_payments = datastore["payments"]

remaining_principal = principal
annual_interest = 0
total_interest = 0

interest_paid_through_date = datetime.strptime(datastore["start_interest_date"], "%Y-%m-%d").date()
next_payment_date = datetime.strptime(datastore["first payment month"], '%Y-%m-%d').date()
next_bill_date = date(year=next_payment_date.year, month=next_payment_date.month, day=payment_day_of_month)
old_bill_date = next_bill_date
payment_number = 1
current_year = next_bill_date.year

print "Payment History:"
print "\tBill\t\t\tPayment\tDays of\tPayment\t\tRemaining\t\tPrincipal\tInterest\t\t\tNew"
print "#\tDate\t\t\tDate\tInterst\tAmount\t\tPrincipal\t\tPayment\t\tPayment\t\t\tBalance"
for payment in actual_payments:
    payment_date = datetime.strptime((payment[0]), '%Y-%m-%d').date()
    payment_amount = Decimal(payment[1]).quantize(Decimal("1.00"))
    #print next_bill_date, "\t", next_payment_date, "\t",
    days_since_last_payment = (payment_date-interest_paid_through_date).days
    #print days_since_last_payment
    new_interest = (days_since_last_payment * remaining_principal * daily_interest_rate).quantize(Decimal("0.00"))
#   new_principal = monthly_payment - new_interest
    new_principal = payment_amount - new_interest

    print payment_number, " ", next_bill_date, "\t\t", payment_date, "\t", days_since_last_payment, "\t", payment_amount, "\t\t\t", remaining_principal, "\t\t", new_principal,\
        "\t\t", new_interest,

    interest_paid_through_date = payment_date
    total_interest = total_interest + new_interest
    annual_interest=annual_interest+new_interest
    remaining_principal = remaining_principal - new_principal
    print "\t\t\t", remaining_principal
    payment_number=payment_number+1
    old_bill_date = next_bill_date

    if old_bill_date.month < 12:
        next_bill_date = date(year=old_bill_date.year, month = old_bill_date.month + 1, day=payment_day_of_month)
        print "Total interest paid to date for", old_bill_date.year, " is", annual_interest
    else:
        next_bill_date = date(year=old_bill_date.year+1, month = 1, day=payment_day_of_month)
        print "Total interest for ", old_bill_date.year, " was ", annual_interest
        annual_interest = 0

print "Total interest paid to date on this loan is", total_interest

#loop over remaining scheduled payments and present estimated amortization
print "\nEstimated future amortization"

while (payment_number <= total_periods) and (remaining_principal > 0):
    print payment_number, "\t",
    payment_number = payment_number+1

    print next_bill_date,

    days_since_last_payment = (next_bill_date-interest_paid_through_date).days
    print days_since_last_payment, "\t",
    new_interest = (days_since_last_payment * remaining_principal * daily_interest_rate).quantize(Decimal("0.00"))

    #make sure the last payment isn't too much
    if new_interest+remaining_principal < monthly_payment:
        monthly_payment = new_interest + remaining_principal

    new_principal = monthly_payment - new_interest

    print monthly_payment, "\t\t\t", remaining_principal, "\t\t", new_principal,\
        "\t\t", new_interest,

    remaining_principal = remaining_principal - new_principal
    print "\t", remaining_principal

    interest_paid_through_date = next_bill_date
    old_bill_date = next_bill_date
    if old_bill_date.month < 12:
        next_bill_date = date(year=old_bill_date.year, month = old_bill_date.month + 1, day=payment_day_of_month)
    else:
        next_bill_date = date(year=old_bill_date.year+1, month = 1, day=payment_day_of_month)

if remaining_principal > 0:
    print 'Balloon payment due:', remaining_principal