title: python try to get next available number
body: <p>From a range of numbers [0:2407] I need to know what are the ones that are already being used.</p>
  <pre><code>arrray [0,1,2,..,2407]
  </code></pre>
  <p>To know the ones already used I have a file that I load with pandas.</p>
  <p>example:</p>
  <pre><code>...| Index |...
  ...|   100 |...
  ...|  1572 |...
  ...|  2046 |...
  ...|  2045 |...
  </code></pre>
  <p>I need to remove from my initial list the ones coming from the file.</p>
  <p>trying to do this in a clean and faster way since the files can be quite big.</p>