title: Specifying column order following groupby aggregation
body: <p>The ordering of my age, height and weight columns is changing with each run of the code.  I need to keep the order of my agg columns static because I ultimately refer to this output file according to the column locations.  What can I do to make sure age, height and weight are output in the same order every time?</p>
  
  <pre><code>d = pd.read_csv(input_file, na_values=[''])
  df = pd.DataFrame(d)
  df.index_col = ['name', 'address']
  
  df_out = df.groupby(df.index_col).agg({'age':np.mean, 'height':np.sum, 'weight':np.sum})
  df_out.to_csv(output_file, sep=',')
  </code></pre>