title: AttributeError: 'NoneType' object has no attribute 'add'
body: <p>I got the following error </p>
  
  <pre><code>AttributeError: 'NoneType' object has no attribute 'add'
  </code></pre>
  
  <p>while I tried this. </p>
  
  <pre><code>not_yet_bought_set = set()
  .
  .
  .
  for value in set_dict.itervalues():
      for item in value:
          not_yet_bought_set = not_yet_bought_set.add(item)
  </code></pre>
  
  <p>I dont't get why I got this error, is it because I always make  not_yet_bought_set new? I make this, because when I only do </p>
  
  <pre><code>not_yet_bought_set.add(item)
  </code></pre>
  
  <p>there wont be all the items from all values. I do not know why. </p>
  
  <p>value are sets and </p>
  
  <pre><code>not_yet_bought_set.union(value)
  </code></pre>
  
  <p>also generate this error</p>
  
  <p>Thanks for any help. </p>