smoother with_parent() failure modes

from sqlalchemy import Column
from sqlalchemy import create_engine
from sqlalchemy import ForeignKey
from sqlalchemy import Integer
from sqlalchemy import String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import declarative_base
from sqlalchemy.orm import relationship
from sqlalchemy.orm import Session
from sqlalchemy.orm import with_parent
Base = declarative_base()


class A(Base):
    __tablename__ = 'a'

    id = Column(Integer, primary_key=True)
    data = Column(String)
    bs = relationship("B", backref="a")


class B(Base):
    __tablename__ = 'b'
    id = Column(Integer, primary_key=True)
    a_id = Column(ForeignKey("a.id"))
    data = Column(String)

e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)

s = Session(e)

s.add(A(bs=[B()]))
s.commit()

a1 = s.query(A).first()
b1 = s.query(B).first()

def test_one():
    print(
        with_parent(a1, B.a).compile(compile_kwargs={"literal_binds": True})
    )

def test_two():
    print(
        with_parent(b1, B.a).compile(compile_kwargs={"literal_binds": True})
)


def test_three():
    print(
        with_parent(a1, A.bs).compile(compile_kwargs={"literal_binds": True})
    )
def test_four():
    print(
        with_parent(b1, A.bs).compile(compile_kwargs={"literal_binds": True})
    )

# additional TODO: relationhips with backwards foreign(), remote(),
# which don't derive from the object at all