title: Pandas lookup from one of multiple columns, based on value
body: <p>I have the following DataFrame:</p>
  
  <pre><code>Date    best    a    b    c    d
  1990    a       5    4    7    2
  1991    c       10   1    2    0
  1992    d       2    1    4    12
  1993    a       5    8    11   6
  </code></pre>
  
  <p>I would like to make a dataframe as follows:</p>
  
  <pre><code>Date    best    value   
  1990    a       5
  1991    c       2
  1992    d       12
  1993    a       5
  </code></pre>
  
  <p>So I am looking to find a value based on another row value by using column names. For instance, the value for 1990 in the second df should lookup "a" from the first df and the second row should lookup "c" (=2) from the first df.</p>
  
  <p>Any ideas?</p>