title: Pandas: add a new column with one single value at the last row of a dataframe
body: <p>My request is simple but I am stuck and do not know why... my dataframe looks like this: </p>
  
  <pre><code>   price  time  
  0      1     3    
  1      3     6    
  2      4     7 
  </code></pre>
  
  <p>What I need to do is to add a new column <code>mkt</code> with only one value equal to 10 at the last row (in my example index = 2). What I have tried:</p>
  
  <pre><code>df.mkt=''
  df.mkt.loc[-1] =10
  </code></pre>
  
  <p>But when I want to see again my dataframe the last row is not updated... I know the answer must be simple but I am stuck? Any idea? thanks!</p>