title: How does Python's seek function work?
body: <p>If I have some file-like object and do the following:</p>
  
  <pre><code>F = open('abc', 'r')
  ...
  loc = F.tell()
  F.seek(loc-10)
  </code></pre>
  
  <p>What does seek do? Does is start at the beginning of the file and read <code>loc-10</code> bytes? Or is it smart enough just to back up 10 bytes?</p>