title: Double-indexed dictionary
body: <p>I want to be able to store and look up values in a dictionary based on two integer values.</p>
  
  <p>So when I look up a value I want to use the keys <code>read_length</code> and <code>min_size</code> to access the element, like so:</p>
  
  <pre><code>number_of_read_lengths[read_length][min_size]
  </code></pre>
  
  <p>I know I can create nested dictionaries, but that is a slight hassle.</p>
  
  <p>Is there a simple way of doing what I want to do?</p>