title: Iterate over items in a Counter by counts
body: <p>Let's say I have a <code>Counter</code> object: <code>Counter({'test': 2, 'this': 1, 'is': 1})</code></p>
  
  <p>I would like to iterate over this object in the following way:</p>
  
  <pre><code>c = Counter({'test': 2, 'this': 1, 'is': 1})
  
  for i,s in my_counter_iterator(c):
      print i, ":", s
  
  &gt;&gt; 1 : ['this', 'is']
  &gt;&gt; 2 : ['test']
  </code></pre>
  
  <p>How do I do that efficiently (this code should run on a web server for each request...)?</p>
  
  <p><em>EDIT</em></p>
  
  <p>I have tried this, but I have the feeling there are more efficient ways. Are there?</p>
  
  <pre><code>from itertools import groupby
  
  for k,g in groupby(sorted(c.keys(), key=lambda x: c[x]),key=lambda x: c[x]):
      print k, list(g)
  
  
  1 ['this', 'is']
  2 ['test']
  </code></pre>