title: How to aggregate dataset by multiple keys in python?
body: <p>I want to summarize dataset, having combined unique key
  (in example - (track, singer).</p>
  
  <p>For example: I have dataset with 3 fields: track, singer, cnt
  I want to aggregate (summarize) data by track &amp; singer, summzrizing cnt field.</p>
  
  <pre><code>sales = [
      {'singer': 'Smash',  'track':  'Cry', 'cnt': 10},
      {'singer': 'Smash',  'track':  'Cry', 'cnt': 11},
      {'singer': 'Smash',  'track':  'Dry', 'cnt': 2},
      {'singer': 'Scooter', 'track': 'Shy', 'cnt': 1},
      {'singer': 'Scooter', 'track': 'Die', 'cnt': 0},
  ]
  </code></pre>
  
  <p>I want have:</p>
  
  <pre><code>sales = [
      {'singer': 'Smash',  'track':  'Cry', 'cnt': 21},   &lt;== summmarized
      {'singer': 'Smash',  'track':  'Dry', 'cnt': 2},
      {'singer': 'Scooter', 'track': 'Shy', 'cnt': 1},
      {'singer': 'Scooter', 'track': 'Die', 'cnt': 0},
  ]
  </code></pre>
  
  <p>Thanks for reply.</p>