title: How to compile all lists in a column into one unique list
body: <p>I have a pandas dataframe as below:</p>
  
  <p><a href="https://i.stack.imgur.com/ZbPhn.png" rel="noreferrer"><img src="https://i.stack.imgur.com/ZbPhn.png" alt="enter image description here"></a></p>
  
  <p>How can I combine all the lists (in the 'val' column) into a unique list (set), e.g. <code>[val1, val2, val33, val9, val6, val7]</code>?</p>
  
  <p>I can solve this with the following code. I wonder if there is an easier way to get all unique values from a column without iterating the dataframe rows?</p>
  
  <pre><code>def_contributors=[]
  for index, row in df.iterrows():
      contri = ast.literal_eval(row['val'])
      def_contributors.extend(contri)
  def_contributors = list(set(def_contributors))
  </code></pre>