Cassandra python驱动程序：合并继承+用户定义的函数==问题？

Question

我从这里使用官方示例：http://datastax.github.io/python-driver/cqlengine/models.html#model-inheritance

将继承和UDF结合使用时，它似乎不起作用。我想念什么？

它会引发KeyError，我不知道为什么，我尝试了很多没有成功的事情

谢谢

# coding=utf-8
from uuid import uuid4

from cassandra.cqlengine import connection
from cassandra.cqlengine.columns import Text, UserDefinedType, UUID, Float
from cassandra.cqlengine.management import sync_table
from cassandra.cqlengine.models import Model

from cassandra.cluster import Cluster
from cassandra.cqlengine.usertype import UserType

cluster = Cluster(['127.0.0.1'])
session = cluster.connect()

session.execute(
    "CREATE KEYSPACE IF NOT EXISTS test WITH replication = {'class':'SimpleStrategy', 'replication_factor' : 1};")


###### Copied from example
# http://datastax.github.io/python-driver/cqlengine/models.html#model-inheritance
# Only thing changed is
# class Pet(Model) changed to Pet(UserType) to nest a Pet in a Person
#
#
class Pet(UserType):

    owner_id = UUID(primary_key=True)
    pet_id = UUID(primary_key=True)
    pet_type = Text(discriminator_column=True)
    name = Text()

    def eat(self, food):
        pass

    def sleep(self, time):
        pass

class Cat(Pet):
    __discriminator_value__ = 'cat'
    cuteness = Float()

    def tear_up_couch(self):
        pass

class Dog(Pet):
    __discriminator_value__ = 'dog'
    fierceness = Float()

    def bark_all_night(self):
        pass


class Person(Model):
    __keyspace__ = 'test'
    name = Text(primary_key=True)
    pet = UserDefinedType(Pet)


# the list of hosts will be passed to create a Cluster() instance
connection.setup(['127.0.0.1'], "test", protocol_version=3)

# ...and create your CQL table
sync_table(Person)

Person.create(
    name='john',
    pet=Dog(
        fierceness=0.11
    )
)