title: Creating a "dictionary of sets"
body: <p>I need to efficiently store data in something that would resemble a "dictionary of sets" e.g. have a dictionary with multiple (unique) values matching each unique key. The source of my data would be a (not very well) structured XML. </p>
  
  <p>My idea is:
  I will look through a number of elements and find keys. If the key does not exist, add it to dictionary, if it already exists, just add a new value in the corresponding key. </p>
  
  <p>And the result would be something like:</p>
  
  <pre><code>{
   'key1': {'1484', '1487', 1488', ...}
   'key2': {'1485', '1486', '1489', ...}
   'key3': {'1490', '1491', '1492', ...}
   ...
  }
  </code></pre>
  
  <p>I need to add new keys on the go.
  I need to push unique values into each set.
  I need to be able to iterate through the whole dictionary.</p>
  
  <p>I am not sure if this is even feasible, but if anybody could push me in the right direction, I would be more than thankful.</p>