title: 'int' object is not iterable for list of lists
body: <p>I've run into an error that I can't quite explain. Here is the code:</p>
  
  <pre><code>board = [[1, 2, 3],[1, 2, 3],[1, 2, 3]]
  col = 0
  col_nums = []
  for rows in board:
      col_nums += rows[col]
  </code></pre>
  
  <p>This gives 'int' object is not iterable error.
  This works though:</p>
  
  <pre><code>for rows in board:
      print(rows[col])
  </code></pre>
  
  <p>I want to end with <code>col_nums = [1, 1, 1]</code>. It doesn't seem like I'm iterating over any integers, just <code>rows</code>, which is a list. I think it might have something to do with <code>+=</code>. </p>