handler404 = 'views.custom404'

 def custom404(request):
    return render(request, 'app/404.html', status=404)

 def custom404(request):
    return HttpResponse('This is error 404', status=404)

 urlpatterns = [
    #include() just relates to another URLconf.
    url(r'^app/', include('app.urls', namespace='app')),
    url(r'^', include('app.urls', namespace='app')),
    url(r'^admin/', admin.site.urls),
    url(r'^login/$', login),
    url(r'^logout/$', views.logout_view),
]

 handler404 = 'app.views.custom404'