title: Repetition in Python for each element of list
body: <p>I have a list <code>a = [3,7,4]</code></p>
  
  <p>I want to generate a list repetition of a sequence generated from each element of list like that:</p>
  
  <p><code>b = [1,1,1,2,2,2,2,2,2,2,3,3,3,3]</code></p>