When is the Django session key generated in a Django application?

Question

When is the Django session key generated in a Django web application? More precisely I don't use Django authentication system. I use an external system. I check whether a session id is present for authentication of new tab. If already exist, application will be loaded in the new tab. Else authentication process will take place. But it always show that a key is present(generated precisely). I want to know which is the exact point where the session id or key is generated(Eg: in session middleware)