title: Access dict key and return None if doesn't exist
body: <p>In Python what is the most efficient way to do this:</p>
  
  <pre><code>my_var = some_var['my_key'] | None
  </code></pre>
  
  <p>ie. assign <code>some_var['my_key']</code> to <code>my_var</code> if <code>some_var</code> contains <code>'my_key'</code>, otherwise make <code>my_var</code> be <code>None</code>.</p>