old_contents: from django.contrib.sites.models import Site
  from django.template import loader, RequestContext
  from django.http import HttpResponse
  from django.core.urlresolvers import reverse, NoReverseMatch
  from django.conf import settings
  from django.views.decorators.cache import cache_page
  
  from robots.models import Rule
  
  USE_SITEMAP = getattr(settings, 'ROBOTS_USE_SITEMAP', True)
  SITEMAP_URL = getattr(settings,'ROBOTS_SITEMAP_URL', False)
  
  def rules_list(request, template_name='robots/rule_list.html', 
          mimetype='text/plain', status_code=200):
      """
      Returns a generated robots.txt file with correct mimetype (text/plain),
      status code (200 or 404), sitemap url (automatically).
      """
      scheme = request.is_secure() and 'https' or 'http'
      current_site = Site.objects.get_current()
      if SITEMAP_URL:
          sitemap_url = SITEMAP_URL
      else:
          try:
              sitemap_url = reverse('django.contrib.sitemaps.views.index')
          except NoReverseMatch:
              try:
                  sitemap_url = reverse('django.contrib.sitemaps.views.sitemap')
              except NoReverseMatch:
                  sitemap_url = None
      if sitemap_url is not None and USE_SITEMAP:
          sitemap_url = "%s://%s%s" % (scheme, current_site.domain, sitemap_url)
      rules = Rule.objects.filter(sites=current_site)
      if not rules.count():
          status_code = 404
      t = loader.get_template(template_name)
      c = RequestContext(request, {
          'rules': rules,
          'sitemap_url': sitemap_url,
      })
      return HttpResponse(t.render(c), status=status_code, mimetype=mimetype)
  
  ROBOTS_CACHE_TIMEOUT = getattr(settings, 'ROBOTS_CACHE_TIMEOUT', None)
  if ROBOTS_CACHE_TIMEOUT:
      rules_list = cache_page(rules_list, ROBOTS_CACHE_TIMEOUT)
new_contents: from django.contrib.sites.models import Site
  from django.template import loader, RequestContext
  from django.http import HttpResponse
  from django.core.urlresolvers import reverse, NoReverseMatch
  from django.conf import settings
  from django.views.decorators.cache import cache_page
  
  from robots.models import Rule
  
  USE_SITEMAP = getattr(settings, 'ROBOTS_USE_SITEMAP', True)
  SITEMAP_URL = getattr(settings,'ROBOTS_SITEMAP_URL', None)
  
  def rules_list(request, template_name='robots/rule_list.html', 
          mimetype='text/plain', status_code=200):
      """
      Returns a generated robots.txt file with correct mimetype (text/plain),
      status code (200 or 404), sitemap url (automatically).
      """
      scheme = request.is_secure() and 'https' or 'http'
      current_site = Site.objects.get_current()
      if SITEMAP_URL:
          sitemap_url = SITEMAP_URL
      else:
          try:
              sitemap_url = reverse('django.contrib.sitemaps.views.index')
          except NoReverseMatch:
              try:
                  sitemap_url = reverse('django.contrib.sitemaps.views.sitemap')
              except NoReverseMatch:
                  sitemap_url = None
      if sitemap_url is not None and USE_SITEMAP:
          sitemap_url = "%s://%s%s" % (scheme, current_site.domain, sitemap_url)
      rules = Rule.objects.filter(sites=current_site)
      if not rules.count():
          status_code = 404
      t = loader.get_template(template_name)
      c = RequestContext(request, {
          'rules': rules,
          'sitemap_url': sitemap_url,
      })
      return HttpResponse(t.render(c), status=status_code, mimetype=mimetype)
  
  ROBOTS_CACHE_TIMEOUT = getattr(settings, 'ROBOTS_CACHE_TIMEOUT', None)
  if ROBOTS_CACHE_TIMEOUT:
      rules_list = cache_page(rules_list, ROBOTS_CACHE_TIMEOUT)
subject: Change default value of SITEMAP_URL to None