title: Python: efficient counting number of unique values of a key in a list of dictionaries
body: <p>There must be a better way of writing this Python code where I have a list of people (people are dictionaries) and I am trying to find the number of unique values of a certain key (in this case the key is called Nationality and I am trying to find the number of unique nationalities in the list of people):</p>
  
  <pre><code>no_of_nationalities = []
  for p in people:
      no_of_nationalities.append(p['Nationality'])
  print 'There are', len(set(no_of_nationalities)), 'nationalities in this list.'
  </code></pre>
  
  <p>Many thanks</p>