Amortization

Python module for calculating information regarding amortization loans

The main classes are Loan and Period.

Period

A Period represents a period in a loan schedule.  A Period consists of 4 instance variables - interest, principal, balance and date (optional).  Each of these instance variables represents their respective values in the period of the loan schedule.
Ex: 
>>> period34 = Period(455.67, 121.7, 150000, datetime.date(2012, 1, 1))
>>> print(period34)
  January, 2012   Interest:  455.67   Principal:  121.70   Balance: 150000.00
## pretty print month and year
>>> period34.monthAndYearStr()
'January, 2012'

Loan

A Loan allows you to create a loan which consists of Periods.  A Loan consists of 5 instance variables - rate, nper, pv, date (optional), and typ (default = 0). 

- Rate is the rate of interest on the Loan.  
Note:  Rate is based on the payment period (i.e. monthly).  If you have an annual rate of 4.5% interest on a Loan paid monthly, then your rate is 0.045/12.

- Nper is the number of periods the Loan the consists of.  
Note:  Nper is based on the payment period (i.e. monthly).  If you have a 30 year loan paid monthly, then there are 30 * 12 (360) payments.

- Pv is the present value of the Loan.  This is the amount of principal that must be paid on the loan.

- Date (optional) is the date the loan starts.  Setting this instance variable will allow for dates to appear in the schedule and period functions.

- Typ (default = 0) is the type of Loan.  0 denotes paying at the end of the period; 1 denotes paying at the beginning of the period.
Note:  This may become deprecated.  I don't know if this variable has any purpose.

Example:

$150,000 Loan with 4.5% interest over 15 years, paid monthly.  Start date is August, 2012. 
Initialization
>>> house = Loan(.045/12, 15*12, 150000, datetime.date(2012, 8, 1))
>>> print(house)
   August, 2012   Rate: 0.00375   Nper: 180   Pv: 150000   Typ: 0


Methods


Loan.pmt
The pmt function calculates the periodic (usually monthly) payment of the Loan
>>> house.pmt()
1147.4899332201949


Loan.period
The period function gets any Period of the Loan (that is to say, it returns a Period object)
>>> print(house.period(147))
  October, 2024   Interest:  137.12   Principal: 1010.37   Balance: 35555.28

Note: If you want to use a date instead of a period, use the periodForDate function.


Loan.schedule / Loan.printSchedule
Returns the schedule for the Loan for a certain numbers of Periods or the entirety of the loan (default)

>>> for period in house.schedule():
...	print(period)
  August, 2012   Interest:  562.50   Principal:  584.99   Balance: 149415.01
September, 2012   Interest:  560.31   Principal:  587.18   Balance: 148827.83
  October, 2012   Interest:  558.10   Principal:  589.39   Balance: 148238.44
…
…
And so on until the last period.

>>> for period in house.schedule(178, 180):
...	print(period)
      May, 2027   Interest:   12.81   Principal: 1134.68   Balance: 2282.13
     June, 2027   Interest:    8.56   Principal: 1138.93   Balance: 1143.20
     July, 2027   Interest:    4.29   Principal: 1143.20   Balance:    0.00

Loan.printSchedule has the same usage, but will print out the periods for you with no need for the for loop.

>>> house.printSchedule(178, 180):
      May, 2027   Interest:   12.81   Principal: 1134.68   Balance: 2282.13
     June, 2027   Interest:    8.56   Principal: 1138.93   Balance: 1143.20
     July, 2027   Interest:    4.29   Principal: 1143.20   Balance:    0.00


Loan.periodForDate/Loan.dateForPeriod
Returns the date or period for the period or date.

>>> house.periodForDate(datetime.date(2020, 5, 1))
94
>>> house.dateForPeriod(94)
datetime.date(2020, 5, 1)


Loan.totalCost / Loan.totalInterest
Returns the total cost/interest of the Loan

>>> house.totalCost()
206548.18797963508
>>> house.totalInterest()
56548.187979635084


Loan.payOffDate
Returns the month and year of the last payment/period

>>> house.payOffDate()
'July, 2027'


Loan.interestPaidSincePeriod / Loan.interestRemainingAfterPeriod
Returns the total amount of interest paid before/remaining after a certain period

>>> house.interestPaidSincePeriod(150)
54624.87546062925
>>> house.interestRemainingAfterPeriod(150)
1923.3125190058345

Thus:

>>> house.interestPaidSincePeriod(150) + house.interestRemainingAfterPeriod(150) == house.totalInterest()
True

Note: If you want to use a date instead of a period, use the periodForDate function.


Loan.principalPaidSincePeriod / Loan.principalRemainingAfterPeriod
Returns the total amount of principal paid before/remaining after a certain period

>>> house.principalPaidSincePeriod(150)
117498.61452239998
>>> house.principalRemainingAfterPeriod(150)
32501.38547760002

Thus:

>>> house.principalPaidSincePeriod(150) + house.principalRemainingAfterPeriod(150) == house.pv
True

Note: If you want to use a date instead of a period, use the periodForDate function.


Loan.totalPaidSincePeriod / Loan.totalRemainingAfterPeriod
Returns the total amount of the loan (including interest) paid before/remaining after a certain period

>>> house.totalPaidSincePeriod(150)
172123.48998302923
>>> house.totalRemainingAfterPeriod(150)
34424.69799660586

Thus these values equate to the total cost of the Loan:

>>> house.totalPaidSincePeriod(150) + house.totalRemainingAfterPeriod(150) == house.totalCost()
True

>>> house.totalInterest() + house.pv == house.totalCost()
True ## Recall pv is the total interest of the Loan

Note: If you want to use a date instead of a period, use the periodForDate function.