title: Pandas union on two columns of set
body: <p>I have two columns in a data frame containing sets.</p>
  
  <p>How do I get a new column where each row contains the union of the items from the respective columns?</p>
  
  <p>For example:</p>
  
  <pre><code>col1 : [{1,2} , {4,5}]
  col2 : [{1,6} , {7,5}]
  union : [{1,2,6}, {4,5,7}]
  </code></pre>
  
  <p>A naive try:</p>
  
  <pre><code>df['union'] = df['col1'].apply(lambda x: x.union(df['col2']))
  </code></pre>
  
  <p>does not work</p>