title: TypeError: unhashable type: 'list' when using built-in set function
body: <p>I have a list containing multiple lists as its elements</p>
  
  <pre><code>eg: [[1,2,3,4],[4,5,6,7]]
  </code></pre>
  
  <p>If I use the built in set function to remove duplicates from this list, I get the error</p>
  
  <pre><code>TypeError: unhashable type: 'list'
  </code></pre>
  
  <p>The code I'm using is</p>
  
  <pre><code>TopP = sorted(set(TopP),reverse=True)
  </code></pre>
  
  <p>Where TopP is a list just like in the e.g. Above</p>
  
  <p>Is this usage of set() wrong? Is there any other way in which I can sort the above list?   </p>