text: from __future__ import unicode_literals
  
  from django import http
  from django.apps import apps
  from django.conf import settings
  from django.contrib.redirects.models import Redirect
  from django.contrib.sites.shortcuts import get_current_site
  from django.core.exceptions import ImproperlyConfigured
  from django.utils.deprecation import MiddlewareMixin
  
  
  class RedirectFallbackMiddleware(MiddlewareMixin):
      # Defined as class-level attributes to be subclassing-friendly.
      response_gone_class = http.HttpResponseGone
      response_redirect_class = http.HttpResponsePermanentRedirect
  
      def __init__(self, get_response=None):
          if not apps.is_installed('django.contrib.sites'):
              raise ImproperlyConfigured(
                  "You cannot use RedirectFallbackMiddleware when "
                  "django.contrib.sites is not installed."
              )
          super(RedirectFallbackMiddleware, self).__init__(get_response)
  
      def process_response(self, request, response):
          # No need to check for a redirect for non-404 responses.
          if response.status_code != 404:
              return response
  
          full_path = request.get_full_path()
          current_site = get_current_site(request)
  
          r = None
          try:
              r = Redirect.objects.get(site=current_site, old_path=full_path)
          except Redirect.DoesNotExist:
              pass
          if r is None and settings.APPEND_SLASH and not request.path.endswith('/'):
              try:
                  r = Redirect.objects.get(
                      site=current_site,
                      old_path=request.get_full_path(force_append_slash=True),
                  )
              except Redirect.DoesNotExist:
                  pass
          if r is not None:
              if r.new_path == '':
                  return self.response_gone_class()
              return self.response_redirect_class(r.new_path)
  
          # No redirect was found. Return the response.
          return response
  