python basic: how to sort a list only by considering the number? : <p>I admit I dont know how to give this question a better title. :-( Anybody has a better idea for the title? </p>

<p>Here is my problem: </p>

<p>I want to sort this list:</p>

<pre><code>a = ['at10', 'at11', 'at12', 'at13', 'at9', 'at1', 'at8']
</code></pre>

<p>Every item from the list begins with two letters and has some numbers.</p>

<p>The desired result is as followings. The list is sorted by numbers.</p>

<pre><code>['at1', 'at8', 'at9', 'at10', 'at11', 'at12', 'at13']
</code></pre>

<p>I tried with <code>sorted(a)</code> and many of its <code>key</code> settings. But I could not get the result. Anybody please help me out? Thanks in advance! </p>
