title: Pandas and Dictionary: How to get all unique values for each key?
body: <p>I want to build a dictionary such that the value in the key-value pair is every unique value for that key.</p>
  <p>Consider this example:</p>
  <pre><code>df = pd.DataFrame({'id': [1, 2, 3, 1, 2, 3], 'vals': ['a1', 'a2', 'a3', 'a2', 'a2a', 'a3a']})
  
  # only yields last entry
  dict(zip(df['id'], df['vals']))
  # results
  {1: 'a2', 2: 'a2a', 3: 'a3a'}
  
  # expected value
  {1: ['a1', 'a2'], 2: ['a2', 'a2a'], 3: ['a3', 'a3a']}
  </code></pre>