title: How to sort namedtuple stored in dict value by tuple value
body: <p>I stored namedtuple as value in a dictionary, like below</p>
  
  <pre><code>AssetList = {}
  Asset = namedtuple('Asset_Desc', 'value weight value_per_unit')
  for i in range(0,items):
      AssetList[i] = Asset(values[i], weights[i], values[i]/weights[i])
  print AssetList
  </code></pre>
  
  <p>I would like to sort the dictionary based on "value_per_unit", so I wrote something like below. But I wonder what should be in 'key' ? I tried itemgetter and attrgetter, but it seems I got the index wrong, I can't figure out how to refer to Asset.value_per_unit. Any idea?</p>
  
  <pre><code>Sorted_AssetList = sorted(AssetList.iteritems(), key=???)
  print Sorted_AssetList
  </code></pre>