Calculate the value of your investments and savings at some point in the future

investment_calculator.py

class Investment(object):
    def cash_invested_by_month(self, month):
        raise NotImplementedError
        
    def value_in_month(self, month):
        raise NotImplementedError

class SingleInvestment(Investment):
    def __init__(self, amount, monthly_interest_rate, month=0):
        self.amount = float(amount)
        self.start_month = month
        self.interest_rate = monthly_interest_rate
        
    def cash_invested_by_month(self, month):
        return self.amount
        
    def value_in_month(self, month):
        duration = month - self.start_month
        return self.amount * (self.interest_rate ** duration)
        
class RecurringInvestment(Investment):
    def __init__(self, contribution, monthly_interest_rate, start_month=0, contribute_every_X_months=1):
        self.contribution = float(contribution)
        self.interest_rate = monthly_interest_rate
        self.start_month = start_month
        self.contribute_every_X_months = contribute_every_X_months
        
    def _month_range_until_month(self, end_month):
        return range(0, end_month - self.start_month)[::self.contribute_every_X_months]
        
    def cash_invested_by_month(self, month):
        return sum(
            self.contribution
            for start_month in self._month_range_until_month(month)
        )
        
    def value_in_month(self, month):
        return sum(
            SingleInvestment(self.contribution, self.interest_rate, month=start_month).value_in_month(month)
            for start_month in self._month_range_until_month(month)
        )

class Savings(object):
    def __init__(self, annual_interest=1.07):
        self.annual_interest = annual_interest
        self.investments = []
        
    @property
    def monthly_interest(self):
        return self.annual_interest ** (1.0 / 12)
        
    @monthly_interest.setter
    def monthly_interest_setter(self, val):
        self.annual_interest = val ** 12.0
        
    def invest(self, amount, every_X_months=1, start_month=0, recurring=False):
        if recurring:
            investment = RecurringInvestment(amount, self.monthly_interest, 
                contribute_every_X_months=every_X_months, 
                start_month=start_month)
        else:
            investment = SingleInvestment(amount, self.monthly_interest, start_month)

        self.investments.append(investment)
    
    def cash_invested_by_month(self, month):
        return sum(i.cash_invested_by_month(month) for i in self.investments)
        
    def value_in_month(self, month):
        return sum(i.value_in_month(month) for i in self.investments)
        
    def print_situation_in_month(self, month):
        invested = self.cash_invested_by_month(month)
        value = self.value_in_month(month)
        print("\nSituation after {} months:".format(month))
        print("  Invested:  ${: >10,.0f}".format(invested))
        print("  Gain:      ${: >+10,.0f} ({:+0.1%})".format(value - invested, (value/invested)-1))
        print("")
        print("Value: ${: >0,.0f}".format(value))




s = Savings(annual_interest=1.07)

# $10k Starting cash
s.invest(10000)

# If you save $1,000 / month
s.invest(1000, recurring=True)

# …and invest $5000 in your IRA each year
s.invest(5000, recurring=True, every_X_months=12)

# value in 10 years (month 120)
s.print_situation_in_month(120)
"""
Situation after 120 months:
  Invested:  $   180,000
  Gain:      $   +85,608 (+47.6%)

Value: $265,608
"""