title: delete all keys except one in dictionary
body: <p>I have a dictionary </p>
  
  <pre><code>lang = {'ar':'arabic', 'ur':'urdu','en':'english'}
  </code></pre>
  
  <p>What I want to do is to delete all the keys except one key.
  Suppose I want to save only <code>en</code> here.
  How can I do it ? (pythonic solution)<br>
  <strong>What I have tried</strong>:</p>
  
  <pre><code>In [18]: for k in lang:
     ....:     if k != 'en':
     ....:         del lang_name[k]
     ....
  </code></pre>
  
  <p>Which gave me the run time error:<code>RuntimeError: dictionary changed size during iteration</code></p>