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    <blockquote> <p>This problem might be related to this question: Python: How do I pass a variable by reference?</p> </blockquote> <p>It is.</p> <blockquote> <p>How can I work around this issue?</p> </blockquote> <p>You need to pass a value that can be modified, and modify it. Numbers cannot be modified: <code>a</code> can be changed from referring to <code>23</code> to referring to <code>42</code> instead, but you cannot cause <code>23</code> to become <code>42</code> or vice-versa.</p> <p>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 <code>list</code> 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 <strong>already a part of <code>self</code></strong>.</p> <pre><code>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 </code></pre> <p>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 <code>dict</code> instead.</p> <p>(A constant 'commission' cost is also unrealistic; a percentage makes more sense.)</p>
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    1. COI implemented a dictionary attribute and it works quite well now. Thanks for your detailed answer.
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