title: In Python, find out number of differences between two ordered lists
body: <p>I want to compare two lists of same length</p>
  
  <pre><code>a = [1, 3, 5, 7, 9]
  b = [1, 2, 5, 7, 3]
  </code></pre>
  
  <p>and find out the number of differences <code>n</code>, in this case it'll be <code>n = 2</code>, and also return an error if the length are not equal. What's the pythonic way of doing this?</p>