title: Pandas DataFrame stack multiple column values into single column
body: <p>Assuming the following DataFrame:</p>
  
  <pre><code>  key.0 key.1 key.2  topic
  1   abc   def   ghi      8
  2   xab   xcd   xef      9
  </code></pre>
  
  <p>How can I combine the values of all the key.* columns into a single column 'key', that's associated with the topic value corresponding to the key.* columns?  This is the result I want:  </p>
  
  <pre><code>   topic  key
  1      8  abc
  2      8  def
  3      8  ghi
  4      9  xab
  5      9  xcd
  6      9  xef
  </code></pre>
  
  <p>Note that the number of key.N columns is variable on some external N.</p>