'Return whether the specified `key` is set.
Arguments:
key (`DjangoKeyValueStore.Key`): The field value to delete
Returns: bool'

@contract(key=DjangoKeyValueStore.Key, returns=bool)
def has(self, key):

    if ((key.scope.user == UserScope.ONE) and (not self.user.is_anonymous())):
        assert (key.user_id == self.user.id)
    if (key.scope not in self.cache):
        return False
    return self.cache[key.scope].has(key)
