title: How to initialize defaultdict with keys?
body: <p>I have a dictionary of lists, and it should be initialized with default keys. I guess, the code below is not good (I mean, it works, but I don't feel that it is written in the pythonic way):</p>
  
  <pre><code>d = {'a' : [], 'b' : [], 'c' : []}
  </code></pre>
  
  <p>So I want to use something more pythonic like <code>defaultict</code>: </p>
  
  <pre><code>d = defaultdict(list)
  </code></pre>
  
  <p>However, every tutorial that I've seen dynamically sets the new keys. But in my case all the keys should be defined from the start. I'm parsing other data structures, and I add values to my dictionary only if specific key in the structure also contains in my dictionary. </p>
  
  <p>How can I set the default keys? </p>