def key_filter(self, key, operator='='):
        self.add_filter('__key__', operator, key)
Return rows in `a` that are not in `b`. E.g.::
dataset/ETHPy150Open GoogleCloudPlatform/gcloud-python/gcloud/datastore/query.py/Query.key_filter--crossover with--dataset/ETHPy150Open alimanfoo/petl/petl/transform/setops.py/complement