title: Summation of specific range in a row of a 2d list in Python
body: <p>I have a 2d list, say a <code>[[1,2,3,4],[5,6,7,8,9]]</code>. I know that I can get summation of first row by <code>sum(a[0])</code>. But what if I need summation of first two or three items. The pseudocode should be like this:<br>
  <code>sum(a[0,1:N]) == 0</code> which means summation of 1 to N elements in row 0.</p>