title: Python dictionary increment
body: <p>In Python it's annoying to have to check whether a key is in the dictionary first before incrementing it:</p>
  
  <pre><code>if key in my_dict:
    my_dict[key] += num
  else:
    my_dict[key] = num
  </code></pre>
  
  <p>Is there a shorter substitute for the four lines above?</p>