title: Check if column value is in other columns in pandas
body: <p>I have the following dataframe in pandas</p>
  
  <pre><code>  target   A       B      C
  0 cat      bridge  cat    brush  
  1 brush    dog     cat    shoe
  2 bridge   cat     shoe   bridge
  </code></pre>
  
  <p>How do I test whether <code>df.target</code> is in any of the columns <code>['A','B','C', etc.]</code>, where there are many columns to check?</p>
  
  <p>I have tried merging A,B and C into a string to use <code>df.abcstring.str.contains(df.target)</code> but this does not work.</p>