title: When can self == None
body: <p>I am looking at a snippet <code>if not self:</code> in <a href="https://codereview.stackexchange.com/a/146386/66848">an answer to another question</a> which implements <code>__nonzero__()</code>.</p>
  
  <p>This gets me wondering: apart from <code>__nonzero__()</code> returning <code>False</code> or the trivial local assignment <code>self = None</code>, are there other situations, in which the conditional <code>if not self</code> is true?</p>