title: Python : Searching a directory for files in a list
body: <h2>Context</h2>
  
  <p>I have a list of filenames (many tens of thousands) that I would like to locate within a directory. For all files that are located they must be copied to a single output folder.</p>
  
  <p>Using Python, what would be, <strong>in your opinion, the most efficient* strategy</strong> for doing this? I'm not looking for a solution, but good strategy to get started.</p>
  
  <h2>To break this down:</h2>
  
  <ul>
  <li>How the list of filenames should be stored and the method of searching if a filename is in the list?</li>
  <li>How to go through an entire directory, folder by folder, and consider each file in tern?</li>
  <li>How to copy the file (least processing time)?</li>
  </ul>
  
  <h2>Caveat</h2>
  
  <p>*efficient in the sense that the script execution should not 'hog' system's resources. Other more important applications maybe running concurrently. </p>
  
  <p>Many thanks!</p>