Difference between reverse() and reverse_lazy() in Django

Question

I understand that we can use reverse() in FBV and reverse_lazy() in CBV. I understand that we have to use reverse_lazy() in CBV as the urls are not loaded when the file is imported (Ref: Reverse_lazy and URL Loading?)

What I don't understand is:

How are the urls loaded when we call reverse from the FBV? As when we import the views at the top of the urls.py in a Django app, urlpatterns list is yet to be evaluated. How does reverse() for FBV work but not for CBV?

Answer 1

Consider these two ways of defining the success_url. The first is commented out, the second is the function:

class NewJobCBV(LoginRequiredMixin, CreateView):     template_name = 'company/job.html'     form_class = newJobForm     # success_url = reverse_lazy('newJob')      def get_success_url(self, **kwargs):         return reverse("newJob") 

@CoffeeBasedLifeform : you are right, class attributes are evaluated on import, I checked after reading your answer. So,

1. If we are using success_url we have to use reverse_lazy().
2. If we are reversing inside a function we can use reverse().

Now it is crystal clear.

Thanks CoffeeBasedLifeform :)

Answer 2

#importme.py def a():     print("FUNCTION HELLO")  class B():     print("CLASS HELLO")        >>> import importme >>> CLASS HELLO 

Edit: The reason: The class creation process involves executing the body of the class.

The class body is executed (approximately) as exec(body, globals(), namespace). [...] Once the class namespace has been populated by executing the class body, the class object is created by calling metaclass(name, bases, namespace, **kwds).

https://docs.python.org/3/reference/datamodel.html?highlight=metaclass#executing-the-class-body

---

---

My original answer text. You can ignore it - I'm just leaving it in because mirek's comment was a direct response to it:

Class attributes are evaluated on import. The answer to when or exactly how that happens, resides within the depths of python's import system.