title: how to flatten a 2D list to 1D without using numpy?
body: <p>I have a list looks like this:</p>
  
  <pre><code>[[1,2,3],[1,2],[1,4,5,6,7]]
  </code></pre>
  
  <p>and I want to flatten it into <code>[1,2,3,1,2,1,4,5,6,7]</code></p>
  
  <p>is there a light weight function to do this without using numpy?</p>