title: Pandas - conditionally select source column of data for a new column based on row value
body: <p>Is there a pandas function that allows selection from different columns based on a condition? This is analogous to a CASE statement in a SQL Select clause. For example, say I have the following DataFrame:</p>
  
  <pre><code>foo = DataFrame(
      [['USA',1,2],
      ['Canada',3,4],
      ['Canada',5,6]], 
      columns = ('Country', 'x', 'y')
  )
  </code></pre>
  
  <p>I want to select from column 'x' when Country=='USA', and from column 'y' when Country=='Canada', resulting in something like the following:</p>
  
  <pre><code>  Country  x  y  z
  0     USA  1  2  1
  1  Canada  3  4  4
  2  Canada  5  6  6
  
  [3 rows x 4 columns]
  </code></pre>