title: Initializing a dictionary in python with a key value and no corresponding values
body: <p>I was wondering if there was a way to initialize a dictionary in python with keys but no corresponding values until I set them. Such as:</p>
  
  <pre><code>Definition = {'apple': , 'ball': }
  </code></pre>
  
  <p>and then later i can set them:</p>
  
  <pre><code>Definition[key] = something
  </code></pre>
  
  <p>I only want to initialize keys but I don't know the corresponding values until I have to set them later. Basically I know what keys I want to add the values as they are found. Thanks.</p>