title: How to remove specific element from sets inside a list using list comprehension
body: <p>I think this may be related to set being <code>mutable</code>.</p>
  <p>Basically, I can remove an element from a set using <code>set.discard(element)</code>. However, <code>set.discard(element)</code> itself returns <code>None</code>. But I'd like to get a copy of the updated set. For example, if I have a list of sets, how can I get an updated copy conveniently using list comprehension operations?</p>
  <p>Sample code:</p>
  <pre><code>test = [{'', 'a'}, {'b', ''}]
  print [x.discard('') for x in test]
  print test
  </code></pre>
  <p>will return</p>
  <pre><code>[None, None]
  [set(['a']), set(['b'])]
  </code></pre>