title: Python Group by more than one keys in List of Dictionary
body: <p>I have the list of dictionary like:</p>
  
  <pre><code>[ {'price': 100,  'product_id': 30089, 'grade_id': 24, 'qty': 1.0},   
    {'price': 200, 'product_id': 30089, 'grade_id': 22, 'qty': 1.0},    
     {'price': 100,  'product_id': 30089, 'grade_id': 24, 'qty': 5.0},   
     {'price': 300, 'product_id': 30993, 'grade_id': 22, 'qty': 1.0} ]
  </code></pre>
  
  <p>Now I want to make group by this List by 2 keys (product_id and grade_id) and the final result will be a new list like:</p>
  
  <pre><code>[ {'price': 200,  'product_id': 30089, 'grade_id': 24, 'qty': 6.0},
    {'price': 200, 'product_id': 30089, 'grade_id': 22, 'qty': 1.0}, 
    {'price': 300, 'product_id': 30993, 'grade_id': 22, 'qty': 1.0}]
  </code></pre>