views.py

    def get_context_data(self, **kwargs):
        context = super().get_context_data(**kwargs)

        filter_argument = self.request.GET.get('filter')
        if filter_argument:
            cleaned_argument = filter_argument.strip()
            if cleaned_argument:
                context['filter'] = cleaned_argument

        context['orderby'] = self.request.GET.get('orderby')

        return context