title: Name is not defined in a list comprehension with multiple loops
body: <p>I'm trying to unpack a complex dictionary and I'm getting a <code>NameError</code> in a list comprehension expression using multiple loops:</p>
  
  <pre><code>a={
    1: [{'n': 1}, {'n': 2}],
    2: [{'n': 3}, {'n': 4}],
    3: [{'n': 5}],
  }
  good = [1,2]
  print [r['n'] for r in a[g] for g in good]
  # NameError: name 'g' is not defined
  </code></pre>