Django isnullfield admin filter

class IsNullFieldListFilter(FieldListFilter):
    def __init__(self, field, request, params, model, model_admin, field_path):
        self.lookup_kwarg = '%s__isnull' % field_path
        self.lookup_remove = '%s__exact' % field_path
        self.lookup_val = request.GET.get(self.lookup_kwarg)
        super().__init__(field, request, params, model, model_admin, field_path)
        if (self.used_parameters and self.lookup_kwarg in self.used_parameters and
                self.used_parameters[self.lookup_kwarg] in ('1', '0')):
            self.used_parameters[self.lookup_kwarg] = bool(int(self.used_parameters[self.lookup_kwarg]))

    def expected_parameters(self):
        return [self.lookup_kwarg]

    def choices(self, changelist):
        for lookup, title in (
                (None, 'All'),
                ('1', 'No'),
                ('0', 'Yes')):

            yield {
                'selected': self.lookup_val == lookup,
                'query_string': changelist.get_query_string({self.lookup_kwarg: lookup,}, remove=[self.lookup_remove]),
                'display': title,
            }