title: Sorting Pandas Dataframe by order of another index
body: <p>Say I have two dataframes, df1 and df2 that share the same index. df1 is sorted in the order that I want df2 to be sorted.</p>
  
  <pre><code>df=pd.DataFrame(index=['Arizona','New Mexico', 'Colorado'],columns=['A','B','C'], data=[[1,2,3],[4,5,6],[7,8,9]])
  print df
  
              A  B  C
  Arizona     1  2  3
  New Mexico  4  5  6
  Colorado    7  8  9
  
  
  df2=pd.DataFrame(index=['Arizona','Colorado', 'New Mexico'], columns=['D'], data=['Orange','Blue','Green'])
  print df2
                   D
  Arizona     Orange
  Colorado      Blue
  New Mexico   Green
  </code></pre>
  
  <p>What is the best / most efficient way of sorting the second dataframe by the index of the first?</p>
  
  <p>One option is just joining them, sorting, and then dropping the columns:</p>
  
  <pre><code>df.join(df2)[['D']]
  
                   D
  Arizona     Orange
  New Mexico   Green
  Colorado      Blue
  </code></pre>
  
  <p>Is there a more elegant way of doing this?</p>
  
  <p>Thanks!</p>