Question

I have this code I am trying out and I can’t figure out how to modify the instance attributes when I use them as parameters when calling a method.

class Portfolio(object):
    def __init__(self):
        self.qtyA = 50
        self.qtyB = 0
        self.qtyC = 0
        self.cash = 0  

    def __str__(self):
        return ("%d %d %d %.2f")%(self.qtyA, self.qtyB, self.qtyC, self.cash)

    def buy(self, stockQty, qtyToBuy, price, commission=20):
        stockQty += qtyToBuy
        print stockQty
        buyTransaction = price * qtyToBuy
        self.cash -= buyTransaction + commission

>>> p = Portfolio()
>>> p.buy(p.qtyA,10,25)
60
>>> print p
50 0 0 -270.00

What seems to be happening is that a new variable stockQty is being created when buy is called. I would expect that a reference to the attribute qtyA would be passed instead of the value. This problem might be related to this question: How do I pass a variable by reference?

How can I work around this issue?

Answer 1

This problem might be related to this question: Python: How do I pass a variable by reference?

It is.

How can I work around this issue?

You need to pass a value that can be modified, and modify it. Numbers cannot be modified: a can be changed from referring to 23 to referring to 42 instead, but you cannot cause 23 to become 42 or vice-versa.

In your case, the natural way to do this is to also notice the other silly thing you’re doing – using a bunch of related, similarly-named variables in parallel – and fix that as well, by replacing them with a sequence. The list type is a modifiable sequence. You need to pass the list itself instead of just a numeric quantity, and indicate which element of the list to replace. Except you don’t actually need to pass the list, because it’s already a part of self.

class Portfolio(object):
    def __init__(self):
        self.quantity = [50, 0, 0]
        self.cash = 0  

    def __str__(self):
        return ("%d %d %d %.2f")%(self.quantity[0], self.quantity[1], self.quantity[2], self.cash)

    def buy(self, stockToBuy, amountToBuy, price, commission=20):
        self.quantity[stockToBuy] += amountToBuy
        cost = price * amountToBuy
        self.cash -= cost + commission

For a more flexible solution, you might consider the idea of having an association between stock names and quantities of stock – after all, who knows what stocks the client might want in the future. We can do this simply by using a dict instead.

(A constant ‘commission’ cost is also unrealistic; a percentage makes more sense.)