title: Group one column by another column in pandas?
body: <p>I would like to get the median value of one column and use the associated value of another column. For example,</p>
  
  <pre><code>   col1  col2 index
  0     1     3     A
  1     2     4     A
  2     3     5     A
  3     4     6     B
  4     5     7     B
  5     6     8     B
  6     7     9     B
  </code></pre>
  
  <p>I group by the index to get the median value of col 1, and use the associated value of col 2 to get</p>
  
  <pre><code>   col1  col2 index
      2     4     A
      5     7     B
  </code></pre>
  
  <p>I can't use the actual median value for index B because it will average the two middle values and that value won't have a corresponding value in col 2. 
  What's the best way to do this? Will a groupby method work? Or somehow use sort? Do I need to define my own function?        </p>