title: Initialize list in a python nested dictionary
body: <p>I am building a dynamic nested python dictonary as below</p>
  
  <pre><code>08-Jul-2016 |Offlines | [value1,value2] 
              |Gobxmls  | [value1,value3,value3,value4] 
              |Bregs    | []
  </code></pre>
  
  <p>During initialization, I am not able to add/append elements in list at the end of this dictionary.
  This is how i have created the nested dict:</p>
  
  <pre><code>class nested_dict(dict):
      def __missing__(self, key):
          value = self[key] = type(self)()
          return value
  
  d = nested_dict()
  </code></pre>
  
  <p>I want to initialize it this way (within a loop) without checking everytime if its a null list. Is it possible ?</p>
  
  <pre><code>for value in values
      d[date][Offlines].append(value)
  </code></pre>