title: Pandas: Add one column's value to another if index exists
body: <p>I have two dataframes:</p>
  
  <p>A:</p>
  
  <pre><code>   foo   bar   foo2  
  0   1     1     1      
  1   2     2     2
  2   3     3     3
  3   3     3     3
  </code></pre>
  
  <p>B:</p>
  
  <pre><code>   foo
  0   1
  2   2
  3   1 
  4   5
  </code></pre>
  
  <p>I would like to add up values of B whose index exists in A to a column of A (on the row with corresponding index) WITHOUT
  values from B which have no corresponding index in A being added or removing indices not present in both A and B:</p>
  
  <pre><code>   foo   bar   foo2  
  0   2     1     1      
  1   2     2     2
  2   5     3     3
  3   6     3     3
  </code></pre>
  
  <p>I feel like this should be straight forward but using <code>add</code> and <code>concat</code> I end up with either not all rows from A or a union of A and B</p>