title: Subtract two strings in python
body: <p>I should calculate the difference between elements two different list. This is my code :</p>
  
  <pre><code>import operator
  a   = ['5', '35.1', 'FFD']
  b    = ['8.5', '11.3', 'AMM']      
  difference = [each[0] - each[1] for each in zip(b, a)]
  print difference
  </code></pre>
  
  <p>I need this output:</p>
  
  <blockquote>
    <p>b-a = ['3.5','-23.8','AMM-FFD']</p>
  </blockquote>
  
  <p>I receive the following error:</p>
  
  <blockquote>
    <p>unsupported operand type(s) for -: 'str' and 'str'</p>
  </blockquote>
  
  <p><strong>I don't want to use any class like <code>numpy</code> or <code>pandas</code></strong></p>