title: Enforcing Class Variables in a Subclass
body: <p>I'm working on extending the Python webapp2 web framework for App Engine to bring in some missing features (in order to make creating apps a little quicker and easier).</p>
  
  <p>One of the requirements here is that each subclass <strong>needs</strong> to have some specific static class variables. Is the best way to achieve this to simply throw an exception if they are missing when I go to utilise them or is there a better way?</p>
  
  <p>Example (not real code):</p>
  
  <p>Subclass:</p>
  
  <pre><code>class Bar(Foo):
    page_name = 'New Page'
  </code></pre>
  
  <p>page_name needs to be present in order to be processed here:</p>
  
  <pre><code>page_names = process_pages(list_of_pages)
  
  def process_pages(list_of_pages)
    page_names = []
  
    for page in list_of_pages:
      page_names.append(page.page_name)
  
    return page_names
  </code></pre>