title: How to groupby the first level index and apply function to the second index in Pandas
body: <p>I have multilevel dataframe 'df' like this :</p>
  
  <pre><code>             col1 col2
  first second
  a        0    5    5
           1    5    5
           2    5    5
  b        0    5    5
           1    5    5
  </code></pre>
  
  <p>And I want to apply a function <code>func</code> (exp: <code>'lambda x: x*10'</code>) to <code>second</code>, somewhat like : </p>
  
  <pre><code>df.groupby(level='first').second.apply(func)
  </code></pre>
  
  <p>and result will lokk like:</p>
  
  <pre><code>             col1 col2
  first second
  a        0    5    5
           10   5    5
           20   5    5
  b        0    5    5
           10   5    5
  </code></pre>
  
  <p>The above command not work for <code>second</code> is not a column, so <code>.second</code> is not accepted by Pandas . </p>
  
  <p>I don't want to do that by <code>df.reset_index()</code> , blablabla..., then finally <code>df.set_index().</code> I prefer to do it in one command, How to do ?</p>