Mortgage Calculator

mortgage.py

"""
Mortgage Calculator for fun
https://en.wikipedia.org/wiki/Mortgage_calculator


What I want to know:
---- What's interest  that I have to pay at payment N ?
---- What's principal that I have to pay at payment N ?

---- What's loan balance before payment N ?
        (P * (1 + r) ** n) - (c * (((1+r)**n - 1)) / r)

---- What's principal that I have paid so far before payment N ?

---- What's interest  that I have paid so far before payment N ?

"""

# VARIABLES
HOME_PRICE       = 200000
DOWN_PERCENTAGE  = 0.0
INTERST_RATE     = 0.03
LOAN_TERM_YEARS  = 30
PAYMENT_PER_YEAR = 12

PROPERTY_TAX     = 0.0130   # California
INSURANCE_RATE   = 0.0022   # California


def cost_of_debt(P, r, N):
    """ Calculate monthly cost to service debt P at a fixed rate r and period N

    Paramters
    =========
        P:  Loan's Principal, the amount of borrowed money
        r:  Annual Interest Rate
        N:  Number of payments over the loan terms 
            e.g 30year * 12 times/ year = 360 payments

    Returns:
    ========
        Monthly payment to service the loan
    """
    if r == 0:
        return P / N
    r = r / 12  # monthly rate
    return (r * P) / (1 - (1 + r)**(-N))



def cost_breakdown(P, r, n, c):
    """Understand how each components in the monthly payment
    
    Parameters
    ==========
        P: Loan principal
        n: payment number
        r: monthly interest
        c: monthly payment (derived from cost of debt)


    Returns:
    ========
        principal, interest paid of Nth payment.
    """
    # Amount owed at end of month N
    beginning_balance = (P * (1 + r) ** n) - (c * (((1+r)**n - 1)) / r)
    interest = beginning_balance * r
    principal = c - interest
    return beginning_balance, principal, interest




MONTHLY_PAYMENT = cost_of_debt(
    P=HOME_PRICE*(1 - DOWN_PERCENTAGE),
    r=INTERST_RATE,
    N=LOAN_TERM_YEARS * PAYMENT_PER_YEAR)

print("Monthly Payment: ${:,.2f}\n".format(MONTHLY_PAYMENT))




COLUMNS = ["YEAR", "MONTH", "BALANCE", "PAYMENT", "PRINCIPAL", "INTEREST"]
print("_______________________________________________________________")
print("{:4} | {:4} | {:10} | {:9} | {:9}  | {:9}|".format(*COLUMNS))
print("===============================================================")
for year_n in range(2):
    cinter, cprinc = 0, 0
    for month_n in range(PAYMENT_PER_YEAR):
        balance, principal, interest = cost_breakdown(
            P=HOME_PRICE * (1-DOWN_PERCENTAGE), 
            n=month_n * (1 + year_n),
            r=INTERST_RATE/12, 
            c=MONTHLY_PAYMENT)
        print("{: <4} | {: <4}  | {:10,.2f} | {: <9,.2f} | {: <9,.2f}  | {: <9,.2f}|".format(
            year_n, month_n, balance, MONTHLY_PAYMENT ,principal, interest, 0
        ))
        cinter += interest
        cprinc += principal
    print("===============================================================")
    r =  INTERST_RATE / 12
    n =  (1 + year_n) * PAYMENT_PER_YEAR
    cumm_interest = (MONTHLY_PAYMENT * (((1+ r)**n - 1)) / r)
    print("Total payment = {:,.2f}".format(MONTHLY_PAYMENT * PAYMENT_PER_YEAR))
    print("Interest      = {:,.2f}".format(cinter))
    print("Principal     = {:,.2f}\n".format(cprinc))
    # print("\nInterest = {:,.2f}\n".format(cumm_interest))