Django Null/Not null admin filter

filter.py

from django.contrib.admin import SimpleListFilter


class NullListFilter(SimpleListFilter):
    def lookups(self, request, model_admin):
        return (
            ('1', 'Null',),
            ('0', '!= Null',),
        )

    def queryset(self, request, queryset):
        if self.value() in ('0', '1'):
            kwargs = {'{0}__isnull'.format(self.parameter_name): self.value() == '1'}
            return queryset.filter(**kwargs)
        return queryset


class StartedNullListFilter(NullListFilter):
    title = u'Started'
    parameter_name = u'started'