title: python class variables and instance variables
body: <p>When I run the following code, it will output <code>20 13</code>, but why? I thought it should be <code>13 13</code>.</p>
  <pre class="lang-py prettyprint-override"><code>class Example(object):
      sum = 10
  
  example = Example()
  Example.sum = 20
  example.sum = 13
  print(Example.sum, example.sum)
  </code></pre>