title: reading a text file columnwise and storing in a list in python
body: <p>I have a text file in the following format:</p>
  
  <pre><code>a,b,c,d,
  1,1,2,3,
  4,5,6,7,
  1,2,5,7,
  6,9,8,5,
  </code></pre>
  
  <p>How can i read it into a list efficiently so as to get the following 
  output?</p>
  
  <pre><code>list=[[1,4,1,6],[1,5,2,9],[2,6,5,8],[3,7,7,5]]
  </code></pre>