ValueError at /admin/login/ substring not found

[dkdndes]

I worked on the upgrade to Django 1.10.5 and Python 3.5 but I am stock now. I get the above error and dont understand why the form is generated inside a template tag but I guess thats why its called shortcut? Any chance you can help?

#from django.views.generic.list_detail import object_list
from django.views.generic import TemplateView
from .models import Subscription, UserSubscription

_formclass = getattr(settings, 'SUBSCRIPTION_PAYPAL_FORM', 'paypal.standard.forms.PayPalPaymentsForm')
_formclass_dot = _formclass.rindex('.') 
_formclass_module = __import__(_formclass[:_formclass_dot], {}, {}, [''])

PayPalForm = getattr(_formclass_module, _formclass[_formclass_dot+1:])
get_paypal_extra_args = Signal(providing_args=['user', 'subscription', 'extra_args'])

My WIP repository (django 1.10.5 and python 3.5) is here:

https://github.com/finnerds/django-paypal-subscription

I tried to add an example and make it more understandable for me what actually happens and how it could be used.