Solution

class Cashflow:
    def __init__(self, val,t,r):
        self.val = val
        self.t = t
        self.r = r
        self.PV = self.valAt(0)
    def valAt(self,time):
        return self.val*(1+self.r)**(time-self.t)
def NPV(arr):
    return sum([x.PV for x in arr])
print(NPV([Cashflow(-1500,0,.05),Cashflow(1000,1,.05),Cashflow(1000,2,.05)]))

Now let’s create an IRR function. Before we do, we need to add something to the cashflow class, a function that finds the present value of the cash flow with a given r.