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])
def pIndex(flows):
    pos = [x for x in flows if x.val > 0]
    neg = [x for x in flows if x.val < 0]
    return NPV(pos)/-NPV(neg)

The first two lines are list comprehension like we have seen except we now have added on an if condition. Basically we get x in our array but only if its value is positive/negative depending on the array. If it does not satisfy the condition it is not added. So we in essence create a filter for both of our arrays. The last line gets the present value of both the positive and negative cash flows, we put a negative sign before the second term so that we can get a positive profitability index measure.

An example:

flows = [Cashflow(-1500,0,.05),Cashflow(1000,1,.05),Cashflow(1000,2,.05)]
print(pIndex(flows))

The rule for taking a project with this measure is that if it is >1 we want to take it. The higher the value, the more appealing that project is because we get more bang for the buck.