title: unpack lists of list into list with individual items
body: <p>I have a list like:</p>
  
  <pre><code>l = [[1,2,3],[4,5]]
  </code></pre>
  
  <p>I would like to unpack each element to make it like:</p>
  
  <pre><code>l = [1,2,3,4,5]
  </code></pre>
  
  <p>I have my solution here:</p>
  
  <pre><code>l = reduce(lambda x, y: x+y, l)
  </code></pre>
  
  <p>Anyone has other Pythonic way? Thanks.</p>