compound.py

#!/usr/bin/python
""" Calculate compounding interest on recurring expenses """

import locale
locale.setlocale(locale.LC_ALL, "en_US")

# User configured parameters
years = 15
interest = .05

# Frequency names. Number of these per year
DAILY = 365.25
WORKDAILY = DAILY * (5.0/7)
WEEKLY = 52.0
MONTHLY = 12.0
YEARLY = 1.0

# List of (Label, Cost, Frequency)
payments = [
    ("drinks", 4, DAILY),
    ("lunches", 15, WORKDAILY),
    ("yogas", 70, MONTHLY),
    ("dinners", 50, WEEKLY),
    ("cars", 20000, YEARLY/5),
]

# Annuity formula: http://www.cs.ucr.edu/~ehwang/interest.html
# pmt=payment per period, i=interest per period, n=no. of periods
def calc(pmt, i, n):
  return pmt*(i+1)*((((1+i)**n)-1)/i)

def fmt(money):
  return locale.currency(money, grouping=True)

def print_cost(label, cost, periods_in_year):
  periods = periods_in_year * years
  total = calc(cost, interest / periods_in_year, periods)
  print("{0} x {1} {2} cost \t{3}".format(
      int(periods * 10) / 10.0,
      fmt(cost),
      label,
      fmt(total)))
  return total

def main():
  total = 0
  print("For {0} years at {1}% return per year:".format(years, interest*100))
  for payment in payments:
      total = total + print_cost(*payment)
  print("For a total of {0}".format(fmt(total)))

if __name__ == '__main__':
  main();