title: Pandas data frame from dictionary
body: <p>I have a python dictionary of user-item ratings that looks something like this:</p>
  
  <pre><code>sample={'user1': {'item1': 2.5, 'item2': 3.5, 'item3': 3.0, 'item4': 3.5, 'item5': 2.5, 'item6': 3.0}, 
  'user2': {'item1': 2.5, 'item2': 3.0, 'item3': 3.5, 'item4': 4.0}, 
  'user3': {'item2':4.5,'item5':1.0,'item6':4.0}}
  </code></pre>
  
  <p>I was looking to convert it into a pandas data frame that would be structured like</p>
  
  <pre><code>     col1   col2  col3
  0   user1  item1   2.5
  1   user1  item2   3.5
  2   user1  item3   3.0
  3   user1  item4   3.5
  4   user1  item5   2.5
  5   user1  item6   3.0
  6   user2  item1   2.5
  7   user2  item2   3.0
  8   user2  item3   3.5
  9   user2  item4   4.0
  10  user3  item2   4.5
  11  user3  item5   1.0
  12  user3  item6   4.0
  </code></pre>
  
  <p>Any ideas would be much appreciated :)</p>