title: suming two dataframes pandas python
body: <p>I have two dataframes</p>
  
  <pre><code>         colA    colB   amount1    amount2     key  
  maindf = xx      yy     12         1           55_ab
           qs      rr     13         2           14_rs
  
          colA    colB   amount1     amount2     key           
  df =    dd      tt     10          5           55_ab
  </code></pre>
  
  <p>I want to add the values of field amount1 and amount 2 of dataframe df to maindf based on field key to get the following</p>
  
  <pre><code>         colA    colB   amount1    amount2     key  
  maindf = xx      yy     22         6           55_ab
           qs      rr     13         2           14_rs
  </code></pre>