title: Flatten a list in python
body: <p>I have a list like this:</p>
  
  <pre><code>[[(video1,4)], [(video2,5),(video3,8)], [(video1,5)], [(video5, 7), (video6,9)]...]
  </code></pre>
  
  <p>each item in this list may contain a single data pair, or a tuple, I want to change this list into </p>
  
  <pre><code>[(video1,4),(video2,5),(video3,8),(video1,5),(video5,7),(video6,9)...]
  </code></pre>
  
  <p>then do this:</p>
  
  <pre><code>for item in list:
      reqs = reqs + item[1]
      b.append(item[0])
  c = set(b)
  </code></pre>
  
  <p>I don't know how to change the list structure, or how to do the same calculation based on the original list?</p>