title: Multi dictionary incrementation
body: <p>I want to create an dictionary in this format:</p>
  
  <pre><code>stats_dict = {
  'num_lines': 9,
  'by_exts': {
      'py': {
          'files': 4            **
          'percent': 60
      },
      'html': {
          'files': 3            **
          'percent': 40
      }
  }}
  </code></pre>
  
  <p>At the 'files' step I can't increment the value. I use the following method in a loop to do the operation.</p>
  
  <pre><code>self.stats_dict['by_exts'][ext_file]['files'] = self.stats_dict['by_exts'][ext_file].setdefault('files', 0) + 1
  </code></pre>
  
  <p>Where is my problem ?</p>