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  1. PO
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    <p>I think you've misunderstood what an "iterator object" is. A <code>for</code> loop is not an iterator object. For all intents and purposes, a for loop like this:</p> <pre><code>myList = [0, 1, 2, 3, 4] for x in myList: print x </code></pre> <p>does this (but more efficiently and less verbosely):</p> <pre><code>i = 0 while i &lt; len(myList) x = myList[i] print x i += 1 </code></pre> <p>So you see, any changes made to <code>x</code> are lost as soon as the next loop starts, because the value of <code>x</code> is overwritten by the value of the next item in the list. </p> <p>As others have observed, it <em>is</em> possible to alter the value of a list while iterating over it. (But don't change its length! That's where you get into trouble.) One elegant way to do so is as follows:</p> <pre><code>for i, x in enumerate(myList): myList[i] = some_func(x) </code></pre> <p><strong>Update</strong>: It's also important to understand that <em>no copying</em> goes on in a for loop. In the above example, <code>i</code> and <code>x</code> -- like all variables in Python -- are more like <em>pointers</em> in C/C++. As the for loop progresses, <code>obj</code> <em>points</em> at <code>myList[0]</code>, <code>myList[1]</code>, etc, in turn. And like a C/C++ pointer, the properties of the object pointed to are not changed when the pointer is changed. But also like a C pointer, you <em>can</em> directly modify the thing pointed at, because it's <strong>not</strong> a copy. In C, this is done by <em>dereferencing</em> the pointer; in Python, this is done by using a mutable object. That's why <a href="https://stackoverflow.com/a/9414474/577088">NPE</a>'s answer works. If <code>i</code> and <code>x</code> were even shallow copies, it wouldn't be possible to do what he does. </p> <p>The reason you can't directly change <code>int</code>s the way you can change <code>list</code>s (as in NPE's answer), is that <code>int</code>s aren't mutable. Once a <code>5</code> object is created, nothing can change its value. That's why passing around a pointer to <code>5</code> is safe in Python -- no side-effects can occur, because the thing pointed to is immutable. </p>
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    1. POModifying a list iterator in Python not allowed?
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    1. USsenderle
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    1. CO+1 for enumerate
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      1. USFred
    2. COObviously, `print a` should be `print myList`, I fixed this. Thanks for the pointer to `enumerate`, very useful, avoids to maintain your own code for the list index!
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      1. USRobW
    3. COTo be even more pythonic & concise, you could do either `[some_func(x) for x in myList]` or `map(some_func, myList)`, although those make copies of the initial list.
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      1. USRyan Madsen
 

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