title: Extend an existing list in Django settings
body: <p>I'm trying to allow a more flexible date input into my application, and would like to use the default Django settings.  Per the documentation, I can define <code>DATE_INPUT_FORMATS</code> in <code>settings.py</code>, but I would like to keep the defaults and add a few others.</p>
  
  <p>I tried extending it as if it already existed that failed miserably:</p>
  
  <pre><code>DATE_INPUT_FORMATS += ['%m-%d-%y',]
  </code></pre>
  
  <p>Yields:  <code>NameError: name 'DATE_INPUT_FORMATS' is not defined</code></p>
  
  <p>I also tried importing the default settings from <code>django.conf</code>, but that is circular, as it tries to include my <code>settings.py</code> that I'm loading it in.</p>
  
  <pre><code>from django.conf import settings as default_settings
  DATE_INPUT_FORMATS = default_settings.DATE_INPUT_FORMATS
  DATE_INPUT_FORMATS += ['%m-%d-%y',]
  </code></pre>
  
  <p>Yields:</p>
  
  <pre><code>Error: Can't find the file 'settings.py' in the directory containing './manage.py'. It appears you've customized things.
  You'll have to run django-admin.py, passing it your settings module.
  (If the file settings.py does indeed exist, it's causing an ImportError somehow.)
  </code></pre>
  
  <p>What is the correct way to extend a default setting in Django's <code>settings.py</code>?</p>