title: Python - intersection between a list and keys of a dictionary
body: <p>I have a list that looks like this:</p>
  
  <pre><code>l1 = ['200:200', '90:728']
  </code></pre>
  
  <p>I have a dictionary that looks like this:</p>
  
  <pre><code>d1 = {'200:200':{'foo':'bar'},'300:300':{'foo':'bar'}}
  </code></pre>
  
  <p>I need to get filter out the dictioary where only the keys are in l1.  The dict should look like this:</p>
  
  <pre><code>result = {'200:200':{'foo':'bar'}}
  </code></pre>
  
  <p>In essence a intersection of a list and the keys of a dict while returning the subsection of the dict.</p>
  
  <p>How do I do this efficiently where time is an issue for a large sets?</p>
  
  <p>Thanks </p>