How to return a default object instead of `None` for an optional relationship in SQLAlchemy?

I have a users table and a roles table, such that a User optionally has a single Role:

class User(Base):
    id: Mapped[int] = mapped_column(primary_key=True)

    role: Mapped[Optional["Role"]] = relationship(back_populates="user")
    # ...


class Role(Base):
    id: Mapped[int] = mapped_column(primary_key=True)
    user_id: Mapped[int] = mapped_column(ForeignKey("user.id"))

    user: Mapped[Optional["User"]] = relationship(back_populates="role")
    # ...

Now in code I would like that User.role always return a Role. If a specific User doesn't have a Role, it should just return a new default one: Role().

How can I replace User.role with some kind of getter function that does this?

A somewhat obvious solution would be:

class User(Base):
    # ...
    @property
    def role_or_new():
        return self.role or Role()

However, I kind of want to override the default attribute name, role.

There are some similarly named existing questions, but I think they're all subtly different:

Flask-SQLAlchemy set relationship default value: solved through events
How to set a default value for a relationship in SQLAlchemy?: Adds a new method, but I want to modify how the existing attribute role behaves
How to set a default value from a SQLAlchemy relationship?: Asks how to get default values based of an existing relationship

Solution

Following @metatoaser's hint, I renamed the relationship from the user and created a property with the original name for compatibility:

class User(Base):
    id: Mapped[int] = mapped_column(primary_key=True)

    role_row: Mapped[Optional["Role"]] = relationship(back_populates="user")
    # ...

    @property
    def role() -> Role:
        if self.role_row is None:
            self.role_row = Role()
            # Consider using `make_transient(self.role_row)` here
        return self.role_row


class Role(Base):
    id: Mapped[int] = mapped_column(primary_key=True)
    user_id: Mapped[int] = mapped_column(ForeignKey("user.id"))

    user: Mapped[Optional["User"]] = relationship(back_populates="role_row")
    # ...

Mind that the back_populates= of the Role model is now different too.

I opted for a public property, role_row, such an advanced user could still choose to use the original relationship, which would be discouraged with a name like _role.

Final note: the newly created Role() will be queued in the session by default! E.g.:

user = session.get(User, 1)
if user.role.access:
    # ...

user.email = new_email
session.commit()

...will trigger the creation of a new Role in the database! This is because every created object by default corresponds to a database row.
If you want to prevent this default object from always appearing in the database on the next commit, you could add a call to make_transient in the custom getter.