title: Pandas conditionally copying of cell value
body: <p>Working with a Pandas DataFrame, I am trying to copy data from one cell into another cell only if the recipient cell contains a specific value. The transfer should go from:</p>
  
  <pre><code>   Col1    Col2
  0     4       X
  1     2       5
  2     1       X
  3     7       8
  4    12      20
  5     3       X
  </code></pre>
  
  <p>And the result should be</p>
  
  <pre><code>   Col1    Col2
  0     4       4
  1     2       5
  2     1       1
  3     7       8
  4    12      20
  5     3       3
  </code></pre>
  
  <p>Is there an elegant or simple solution I am missing?</p>