title: get keys correspond to a value in dictionary
body: <p>I have this dictionary in python </p>
  
  <pre><code>d={1: 'a', 2: 'b', 3: 'c'}
  </code></pre>
  
  <p>with d[1] I get </p>
  
  <pre><code>&gt;&gt;&gt; d[1]
  'a'
  </code></pre>
  
  <p>how I can get the key correspond to a value ?<br>
  example : with 'a' get 1 </p>