title: Append nested dictionaries
body: <p>If you have a nested dictionary, where each 'outer' key can map to a dictionary with multiple keys, how do you add a new key to the 'inner' dictionary? For example, I have a list where each element is composed of 3 components: an outer key, an inner key, and a value (A B 10).</p>
  
  <p>Here's the loop I have so far</p>
  
  <pre><code>for e in keyList:
      nested_dict[e[0]] = {e[2] : e[3:]}
  </code></pre>
  
  <p>Right now, instead of adding the new key:value to the inner dictionary, any new key:value outright replaces the inner dictionary.</p>
  
  <p>For instance, lets say the keyList is just [(A B 10), (A D 15)]. The result with that loop is </p>
  
  <pre><code>{'A' : {'D' : 15}}
  </code></pre>
  
  <p>How can I make it so that instead it's</p>
  
  <pre><code>{'A' : {'B' : 10, 'D' : 15}}
  </code></pre>