title: pandas add a column with only one row
body: <p>This sounds a bit weird, but I think that's exactly what I needed now:</p>
  
  <p>I got several pandas dataframes that contains columns with float numbers, for example:</p>
  
  <pre><code>   a  b  c
  0  0  1  2
  1  3  4  5
  2  6  7  8
  </code></pre>
  
  <p>Now I want to add a column, with only one row, and the value is equal to the average of column 'a', in this case, is 3.0. So the new dataframe will looks like this:</p>
  
  <pre><code>   a  b  c  average
  0  0  1  2  3.0
  1  3  4  5
  2  6  7  8
  </code></pre>
  
  <p>And all the rows below are empty. </p>
  
  <p>I've tried things like <code>df['average'] = np.mean(df['a'])</code> but that give me a whole column of 3.0. Any help will be appreciated.</p>