views
class StatisticsListAndAdd(PurchaseWeek, View):
    """Отображение Формы, Статистики и Фильтра в одном шаблоне"""

    form = get_form_week
    queryset = shopping_filter_calendar
    template_name = 'statistics/statistics.html'

utils
class PurchaseWeek:
    """Миксин для отображения списка покупок, формы для их записи и фильтра по дате"""
    form = None
    queryset = None
    template_name = None

    def get(self, request):
        """Получаем Форму и Покупки"""
        try:
            return render(request, self.template_name, {'form': self.form, 'statistics': self.queryset})
        except AttributeError:
            pass

    def post(self, request):
        """ПОСТ запрос для записи покупки в БД"""
        return get_purchases_for_week(self, request)

main_logic
def get_form_week(self):
    date = datetime.date.today()
    start_week = date - datetime.timedelta(date.weekday())
    end_week = start_week + datetime.timedelta(7)
    week = {
        'start_of_week': start_week,
        'end_of_week': end_week,
    }
    form = PurchaseForWeek(self.request.GET.get('form'), initial=week)
    return form


def get_purchases_for_week(self, request):
    date = datetime.date.today()
    start_week = date - datetime.timedelta(date.weekday())
    end_week = start_week + datetime.timedelta(7)
    week = {
        'start_of_week': start_week,
        'end_of_week': end_week,
    }
    form = PurchaseForWeek(request.POST)
    qs = get_users_purchases(self)

    if form.is_valid():
        form.save()
        qs = qs.filter(date__range=form)
        return form
    return qs

Ошибка
The view statistics_m.views.StatisticsListAndAdd didn't return an HttpResponse object. It returned None instead.