title: One line iteration through dictionaries in dictionaries
body: <pre><code>G2 = {'a': {'c': 1, 'b': 1}, 'b': {'a': 1, 'c': 1}}
  
  b = G2.values()
  
  for i in b:
      for key, value in i.items():
          list.append(key)
  
  #result: ['c', 'b', 'a', 'c']
  </code></pre>
  
  <p>Can I get the same result but using a list generator? 
  I tried it like this: </p>
  
  <pre><code>list2 = [key for key, value in i.items() for i in b]
  
  #but i get: ['a', 'a', 'c', 'c']
  </code></pre>