title: How do I merge a 2D array in Python into one string with List Comprehension?
body: <p>List Comprehension for me seems to be like the opaque block of granite that regular expressions are for me. I need pointers.</p>
  
  <p>Say, I have a 2D list:</p>
  
  <pre><code>li = [[0,1,2],[3,4,5],[6,7,8]]
  </code></pre>
  
  <p>I would like to merge this either into one long list</p>
  
  <pre><code>li2 = [0,1,2,3,4,5,6,7,8]
  </code></pre>
  
  <p>or into a string with separators:</p>
  
  <pre><code>s = "0,1,2,3,4,5,6,7,8"
  </code></pre>
  
  <p>Really, I'd like to know how to do both.</p>