old_contents: from django.views.generic.edit import FormMixin
  from django.contrib import messages
  
  
  class SuccessMessageMixin(FormMixin):
      """
      Adds a success message on successful form submission.
      """
      success_message = ''
  
      def form_valid(self, form):
          response = super(SuccessMessageMixin, self).form_valid(form)
          success_message = self.get_success_message(form.cleaned_data)
          if success_message:
              messages.success(self.request, success_message)
          return response
  
      def get_success_message(self, cleaned_data):
          return self.success_message % cleaned_data
  
new_contents: from django.contrib import messages
  
  
  class SuccessMessageMixin(object):
      """
      Adds a success message on successful form submission.
      """
      success_message = ''
  
      def form_valid(self, form):
          response = super(SuccessMessageMixin, self).form_valid(form)
          success_message = self.get_success_message(form.cleaned_data)
          if success_message:
              messages.success(self.request, success_message)
          return response
  
      def get_success_message(self, cleaned_data):
          return self.success_message % cleaned_data
  
subject: Remove unnecessary and problematic parent class from SuccessMessageMixin