title: python unflatten list based on another list
body: <p>I have two lists :</p>
  
  <pre><code>list_1 = [[1,2],[3,4,5]]
  list_2 = ['a','b','c','d','e']
  </code></pre>
  
  <p>list_1 and list 2 are always the same length</p>
  
  <p>I want to format list_2 and unflatten it using list_1 (positions of elements)</p>
  
  <p>desired output :</p>
  
  <pre><code>list_2_formatted = [[a,b], [c,d,e]]
  </code></pre>