title: create new rank column in python , or use sort and reset index rank - how to?
body: <p>I'm trying to create a rank column ( descending ) based on value in 'a' to create 'rank'
  Here's below is what I got after sorted , but now I have index as where I want as my column 'rank' how can I use index to create a new variable ? </p>
  
  <p>or is there a function rank() that I can easily use in python to get descending ranked based on column 'a' ?</p>
  
  <pre><code>df = DataFrame(rand(10, 2), columns=list('ab'))
  df.sort('a',ascending = False).reset_index()
  # df.reset_index()
  </code></pre>