Python iterator returning unwanted 'None'

Question

Why is my iterator returning extra \'None\' in the output. For the parameters/example below, I am getting [None,4,None] instead of the desired [4] Ca

Answer 1

Functions return None if you don't have an explicit return statement. That's what happens in __next__ when if (self.purchase[old])%(self.d) == 0: isn't true. You want to stay in your __next__ until it has a value to return.

class Prizes(object):
    def __init__(self,purchase,n,d):
        self.purchase = purchase
        self.length = len(purchase)
        self.i = n-1
        self.n = n
        self.d = d

    def __iter__(self):
        return self

    def __next__(self):
        while self.i < self.length:
            old = self.i
            self.i += self.n
            if (self.purchase[old])%(self.d) == 0:
                return old+1
        raise StopIteration

def superPrize(purchases, n, d):
    return list(Prizes(purchases, n, d))

purchases = [12, 43, 13, 465, 1, 13]
n = 2
d = 3
print(superPrize(purchases, n, d))