title: Get a part of string Python
body: <p>I have two columns of a csv file like</p>
  
  <pre><code>a aa
  b bbb
  c b
  d nn
  ee e
  </code></pre>
  
  <p>I'd like to get something like this</p>
  
  <pre><code>- a
  - bb
  c b
  d nn
  e -
  </code></pre>
  
  <p>I tried <code>col1 not in col2</code> and it's returning me true or false. </p>