This produces an AssertionError. Why?

base.py

from sqlalchemy.ext.declarative import declarative_base, declared_attr
from sqlalchemy import Column, Integer, String, create_engine

Base = declarative_base()

class Thing(Base):
    __tablename__ = 'things'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    discriminator = Column('type', String)
    @declared_attr
    def __mapper_args__(cls):
        args = {}
        args['polymorphic_on'] = 'discriminator'
        args['polymorphic_identity'] = cls.__name__.lower()
        return args

    def __init__(self, name):
        self.name = name

createthings.py

import db
import base
import otherthings


base.Base.metadata.drop_all(base.engine)
base.Base.metadata.create_all(base.engine)
things = [otherthings.Foo('hoohaa'), otherthings.Bar('woop woop')]
for thing in things:
    db.session.add(thing)
db.session.commit()

db.py

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
engine = create_engine('sqlite:///test.db', echo=True)

Session = sessionmaker(bind=engine)
session = Session()

operator.py

from base import Thing
#from otherthings import *  # Need this or AssertionError
import db

print db.session.query(Thing).all()

otherthings.py

from base import Thing

class Foo(Thing):
    def custom_behaviour(self):
        print self.name

class Bar(Thing):
    def calculate_stuff(self):
        print self.name * 3