Future value calculation with a increasing amount of contribution annually, up to X amount.

future_value.py

import numpy as np
import locale

locale.setlocale(locale.LC_ALL, '')

def sliding_payment_fv(start_pmt=100, annual_pmt_increase=100, max_pmt=300,
                 years=10, interest_rate=0.05, present_value=100):
    future_value = 0
    pmt = start_pmt
    for year in range(years):   
        yearly_increase = np.fv(interest_rate/12, 12, -pmt, -present_value)
        future_value += yearly_increase
        print "amount of contribution / month for year %s: %d" % (year+1, pmt)
        print "value for year %d %s" % (year+1, 
                    locale.currency(future_value, grouping=True))
        if pmt < max_pmt:
            pmt += annual_pmt_increase

if __name__ == "__main__":
    sliding_payment_fv()

gistfile1.txt

$ python future_value.py
amount of contribution / month for year 1: 100
value for year 1 $1,333.00
amount of contribution / month for year 2: 200
value for year 2 $3,893.89
amount of contribution / month for year 3: 300
value for year 3 $7,682.66
amount of contribution / month for year 4: 300
value for year 4 $11,471.43
amount of contribution / month for year 5: 300
value for year 5 $15,260.21
amount of contribution / month for year 6: 300
value for year 6 $19,048.98
amount of contribution / month for year 7: 300
value for year 7 $22,837.75
amount of contribution / month for year 8: 300
value for year 8 $26,626.53
amount of contribution / month for year 9: 300
value for year 9 $30,415.30
amount of contribution / month for year 10: 300
value for year 10 $34,204.07