reverse for success_url on Django Class Based View complain about circular import

Question

When using method-based view, redirecting with reverse didn't complain about this and can still find the root url conf. But, in class-based views, it complain:

ImproperlyConfigured at /blog/new-post/  The included urlconf 'blog.urls' does not appear to have any patterns in it. If you see valid patterns in the file then the issue is probably caused by a circular import. 

My class is defined like this:

class BlogCreateView(generic.CreateView):     form_class = Blog     template_name = 'blog/new-post.html'     success_url = reverse('blog:list-post') 

How to properly use reverse for success_url in class-based views? Thanks.

PS: And I'm interested in why it's need to restart runserver after this error (not like an error like TemplateDoesNotExists which is no need to restart runserver)

Answer 1

Using reverse in your method works because reverse is called when the view is run.

def my_view(request):     url = reverse('blog:list-post')     ... 

If you overrride get_success_url, then you can still use reverse, because get_success_url calls reverse when the view is run.

class BlogCreateView(generic.CreateView):     ...     def get_success_url(self):         return reverse('blog:list-post') 

However, you can't use reverse with success_url, because then reverse is called when the module is imported, before the urls have been loaded.

Overriding get_success_url is one option, but the easiest fix is to use reverse_lazy instead of reverse.

from django.urls import reverse_lazy # from django.core.urlresolvers import reverse_lazy  # old import for Django < 1.10  class BlogCreateView(generic.CreateView):     ...     success_url = reverse_lazy('blog:list-post') 

To answer your final question about restarting runserver, the ImproperlyConfigured error is different from TemplateDoesNotExists because it occurs when the Django application is loaded.