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copied!<p>You <strong>appended</strong> <code>bList</code> to <code>aList</code>, then popped it off again and appended <code>aList</code> to <code>bList</code>.</p> <p>Here is what happens step by step:</p> <ol> <li><p><code>aList.append(bList)</code> adds <code>bList</code> as a single value to <code>aList</code>; <code>aList</code> is now:</p> <pre><code>>>> aList=[1,3,"Hello","Barney"] >>> bList=[5,5,5] >>> aList.append(bList) >>> aList [1, 3, 'Hello', 'Barney', [5, 5, 5]] </code></pre> <p>note the <em>nested</em> list; <code>list.append()</code> adds the argument as a single entry in the target list.</p></li> <li><p>You then test if <code>5</code> is in <code>aList</code>; it is not, it is in a <em>nested</em> list:</p> <pre><code>>>> 5 in aList False >>> 5 in aList[-1] True </code></pre></li> <li><p>The <code>else</code> branch uses <code>list.pop()</code> to remove the last element, which is a whole nested list, and appends <code>aList</code> to it; <code>bList</code> still refers to that last list:</p> <pre><code>>>> temp = aList.pop() >>> temp [5, 5, 5] >>> temp is bList True >>> temp.append(aList) >>> bList [5, 5, 5, [1, 3, 'Hello', 'Barney']] </code></pre></li> </ol> <p>You probably wanted to <strong>extend</strong> <code>aList</code> instead, adding just the <em>elements</em> of <code>bList</code> to <code>aList</code>:</p> <pre><code>>>> aList=[1,3,"Hello","Barney"] >>> bList=[5,5,5] >>> aList.extend(bList) >>> aList [1, 3, 'Hello', 'Barney', 5, 5, 5] </code></pre> <p>Now <code>5 in aList</code> is <code>True</code>, and <code>bList</code> is not going to be affected.</p>

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