SQLAlchemy - create an instance in another instances __init__

Newish to SQLAlchemy (so my terminology may be a bit off). I want to create a database object inside the constructor of another, but the problem is I can't add said object to the session, so I get an error.

My schema looks a bit like the following:

class Tag(Base):
    __tablename__ = 'tag'

    id = Column(Integer, Sequence('tag_id_seq'), primary_key=True, nullable=False)
    type = Column(String(1), nullable=False)
    name = Column(String(255), unique=True, nullable=False)

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

    def __repr__(self):
        return "<Tag('%s')>" % (self.id)


class Venue:
    __tablename__ = 'venue'

    tag_id = Column(Integer)
    tag_type = Column(String(1), nullable=False)
    location = Column(String(255), nullable=False)

    tag = ForeignKeyConstraint(
            (tag_id, tag_type),
            (Tag.id, Tag.type),
            onupdate='RESTRICT',
            ondelete='RESTRICT',
            )

    def __init__(self,name,location):
        self.tag = Tag('V',name)
        self.location = location

When I do the following:

session.add(Venue("Santa's Cafe","South Pole"))

I get an error:

UnmappedInstanceError: Class '__builtin__.instance' is not mapped

I assume this is because the the Tag object created in Venue's constructor is not added to the session. My question is how/when do I do this. (I'd really prefer to create that Tag object in the constructor if possible). I think I could do this with a scoped_session but that seems like a really bad solution.

Thanks