 <p>You can't append to a <code>tuple</code> at all (<code>tuple</code>s are <em>immutable</em>), and extending to a <code>list</code> with <code>+</code> requires another list.</p> <p>Make <code>curveList</code> a <code>list</code> by declaring it with:</p> <pre><code>curveList = [] </code></pre> <p>and use:</p> <pre><code>curveList.append(curve) </code></pre> <p>to add an element to the end of it. Or (less good because of the intermediate <code>list</code> object created):</p> <pre><code>curveList += [curve] </code></pre>