title: Why is there an f in the parameters of this method: os.listdir(f'{PATH}valid')?
body: <p>I was taking the fast.ai course and came across this snippet of code (in Python 3):</p>
  
  <pre><code>PATH = "data/dogscats/"
  os.listdir(f'{PATH}valid')
  </code></pre>
  
  <p>This returns the list of the files in the directory <code>data/dogscats/valid</code>, like I expected. However, I don't understand what purpose the "f" in front of <code>'{PATH}valid'</code> serves. When removed, the code throws a "FileNotFound" error. Why is the "f" there? It's not even part of the string? I know this might be an elementary question, but it's something I'd love to understand.</p>
  
  <p>Thank you in advance!</p>