title: List of dicts: Add up numbers with same values
body: <p>I got the following list of dicts</p>
  <pre><code>list_of_dicts = [
                  {'product': 'car', 'city': 'new york', 'quantity': 13},
                  {'product': 'car', 'city': 'new york', 'quantity': 25},
                  {'product': 'bus', 'city': 'miami', 'quantity': 5},
                  {'product': 'container', 'city': 'atlanta', 'quantity' 5},
                  {'product': 'container', 'city': 'atlanta', 'quantity' 8}
              ]
  </code></pre>
  <p>My target is, when values of 'product' and 'city' are the same, sum up the values of 'quantity'. 
  The result should look like this:</p>
  <pre><code>result_list_of_dicts = [
                  {'product': 'car', 'city': 'new york', 'quantity': 38},
                  {'product': 'bus', 'city': 'miami', 'quantity': 5},
                  {'product': 'container', 'city': 'atlanta', 'quantity' 13},
              ]
  </code></pre>
  <p>Is there a pythonic way? I tried a couple of things but I better not show them because they are really ugly.</p>
  <p>Thank you in advance!</p>