title: Python Pandas dataframe: Collect values of a column
body: <p>I have the following data frame:</p>
  
  <pre><code>      var_1     var_2                         item_list 
  0         0         1         [beer, apple, pear, rice]    
  1         0         1          [egg, banana, oil, pear]   
  2         0         1                    [beer, noodle]    
  3         1         0                    [tomato, milk]    
  4         1         0                           [apple] 
  </code></pre>
  
  <p>Is it possible to collect all items in the item_list using data-frame apply function? The output should be something like <code>[beer, apple, pear, rice, egg, banana, oil, pear, ...]</code> without duplicates in the list. </p>
  
  <p>Or I have to iterate cell by cell to collect all values in to one list? </p>