title: Pandas - combine two columns
body: <p>I have 2 columns, which we'll call <code>x</code> and <code>y</code>. I want to create a new column called <code>xy</code>:</p>
  
  <pre><code>x    y    xy
  1         1
  2         2
  
       4    4
       8    8
  </code></pre>
  
  <p><em>There shouldn't be any conflicting values, but if there are, y takes precedence. If it makes the solution easier, you can assume that <code>x</code> will always be <code>NaN</code> where <code>y</code> has a value.</em></p>