title: Generate a dataframe from list with different length
body: <p>Here I got many list with different length, like <code>a=[1,2,3]</code> and <code>b=[2,3]</code></p>
  
  <p>I would like to generate a pd.DataFrame from them, by padding <code>nan</code> at the end of list, like this:</p>
  
  <pre><code>   a  b
  1  1  2 
  2  2  3
  3  3  nan
  </code></pre>
  
  <p>Any good idea to help me do so?</p>