title: Create a defaultdict with a default of zero (0)
body: <p>Similarly to <a href="https://stackoverflow.com/questions/10923334/setting-the-default-string-value-of-pythons-collections-defaultdict">this question</a> I'd like to create a <code>collections.defaultdict</code> with a default value of zero for counting elements of a list of tuples:</p>
  
  <pre><code>tuples = [('foo', 5), ('foo', 5), ('bar', -5), ('bar', -2)]
  totals = defaultdict(0)
  for t in tuples:
      totals[t[0]] += t[1]
  </code></pre>
  
  <p>But <code>defaultdict(0)</code> says:</p>
  
  <pre><code>TypeError: first argument must be callable
  </code></pre>
  
  <p>How am I supposed to create a "callable" zero?</p>