title: Merge Only When Value is Empty/Null in Pandas
body: <p>I have two dataframes in Pandas which are being merged together df.A and df.B, df.A is the original, and df.B has the new data I want to bring over.  The merge works fine and as expected I get two columns col_x and col_y in the merged df.</p>
  
  <p>However, in some rows, the original df.A has values where the other df.B does not.  My question is, <strong>how can I selectively take the values from col_x and col_y and place them into a new col such as col_z ?</strong></p>
  
  <p>Here's what I mean, how can I merge df.A:</p>
  
  <pre><code>date   impressions    spend    col
  1/1/15 100000         3.00     ABC123456
  1/2/15 145000         5.00     ABCD00000
  1/3/15 300000         15.00    (null)
  </code></pre>
  
  <p>with df.B</p>
  
  <pre><code>date    col
  1/1/15  (null)
  1/2/15  (null)
  1/3/15  DEF123456
  </code></pre>
  
  <p>To get:</p>
  
  <pre><code>date   impressions    spend    col_z
  1/1/15 100000         3.00     ABC123456
  1/2/15 145000         5.00     ABCD00000
  1/3/15 300000         15.00    DEF123456
  </code></pre>
  
  <p>Any help or point in the right direction would be really appreciated!</p>
  
  <p>Thanks</p>