title: Are Python sets mutable?
body: <p>Are sets in Python mutable?</p>
  
  <hr>
  
  <p>In other words, if I do this:</p>
  
  <pre><code>x = set([1, 2, 3])
  y = x
  
  y |= set([4, 5, 6])
  </code></pre>
  
  <p>Are <code>x</code> and <code>y</code> still pointing to the same object, or was a new set created and assigned to <code>y</code>?</p>