title: DataFrame.sum returns Series and not a number
body: <p>My basic task is to take vector <code>x=[x1,x2,x3,x4]</code> (which in my case is presented by a row of a Pandas dataframe, lets say a row with an index = 1), multiply it by scalar <code>k</code> and to sum up the results -> <code>x1*k + x2*k + x3*k + x4*k</code>.</p>
  
  <p>I did not find a function that would do it in one step (Is there such a function/operation?), so i do it in two steps. First i multiply my vector <code>x</code> by scalar <code>k</code>, and then i sum up the results:</p>
  
  <pre><code>x_by_k = my_df.loc[[1]]*k
  sum = x_by_k.sum(axis=1)
  </code></pre>
  
  <p>One of the problems i have here is that the resulting <code>sum</code> is of <code>Series</code> type, although effectively it is a number. </p>
  
  <p>Is there a way to perform this sum operation with a number as an output?</p>
  
  <p>Can i do the above described in one step?</p>