title: Why do some includes in Django need strings, and others variable names?
body: <p>Referring to the <a href="http://www.djangobook.com/en/2.0/chapter03.html">Django Book, chapter 3</a>:</p>
  
  <pre><code>from django.conf.urls import patterns, include, url
  
  # Uncomment the next two lines to enable the admin:
  # from django.contrib import admin
  # admin.autodiscover()
  
  urlpatterns = patterns('',
      # Examples:
      # url(r'^$', 'mysite.views.home', name='home'),
      # url(r'^mysite/', include('mysite.foo.urls')),
      # url(r'^admin/doc/', include('django.contrib.admindocs.urls')),
      # url(r'^admin/', include(admin.site.urls)),
  )
  </code></pre>
  
  <p>What determines why one thing is included as a string, and another as regular variable? Why <code>admin.site.urls</code> but not <code>'admin.site.urls'</code>? All other <code>include</code>s are included as strings... I see no logical pattern here.</p>