title: Combine multiple separate lists into a list of lists
body: <p>Below are three lists which I would like to combine into a single nested list:</p>
  
  <pre><code>List_1=[1,2,3]
  List_2=[4,5,6]
  List_3=[7,8,9]
  </code></pre>
  
  <p>My attempt:</p>
  
  <pre><code>List_x=[]
  List_x.append(List_1)
  List_x.append(List_2)
  List_x.append(List_3)
  print List_x
  </code></pre>
  
  <p>Result: </p>
  
  <pre><code>[[1,2,3],[4,5,6],[7,8,9]]
  </code></pre>
  
  <p>Desired result: Same as the result I got, but the approach is extremely slow given the size of my actual data.</p>