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copied!<p>EVERYTHING in python are objects, and python never makes copies unless explicity asked to do so.</p> <p>When you do </p> <pre><code>innerList = [0] * 10 </code></pre> <p>you create a list with 10 elements, <strong>all of them refering to the same <code>int</code> object <em><code>0</code></em></strong>.</p> <p>Since integer objects are <strong>immutable</strong>, when you do</p> <pre><code>innerList[1] = 15 </code></pre> <p>You are changing the second element of the list so that it refers to another integer <em><code>15</code></em>. That always works because of <em><code>int</code></em> objects immutability.</p> <p>That's why</p> <pre><code>outerList = innerList * 5 </code></pre> <p>Will create a <code>list</code> object with 5 elements, each one is a reference to <strong>the same <code>innerList</code></strong> just as above. But since <code>list</code> objects are <strong>mutable</strong>:</p> <pre><code>outerList[2].append('something') </code></pre> <p>Is the same as:</p> <pre><code>innerList.append('something') </code></pre> <p>Because they are two references to the <strong>same <code>list</code> object</strong>. So the element ends up in that single <code>list</code>. It appears to be duplicated, but the fact is that there is only one <code>list</code> object, and many references to it.</p> <p>By contrast if you do</p> <pre><code>outerList[1] = outerList[1] + ['something'] </code></pre> <p>Here you are <strong>creating <em>another</em> <code>list</code> object</strong> (using <code>+</code> with lists is an explicit copy), and assigning a reference to it into the second position of <code>outerList</code>. If you "append" the element this way (not really appending, but creating another list), <code>innerList</code> will be unaffected.</p>

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