SQLAlchemy singleton model

singleton.py

from sqlalchemy.orm.exc import NoResultFound
from sqlalchemy.exc import IntegrityError
from rates.extensions import db


class LoadSingleton:
    @classmethod
    def load(cls):
        id_ = {"id": 1}
        try:
            return db.session.query(cls).filter_by(**id_).one()
        except NoResultFound:
            try:
                with db.session.begin_nested():
                    instance = cls(**id_)
                    db.session.add(instance)
                db.session.commit()
                return instance
            except IntegrityError:
                return db.session.query(cls).filter_by(**id_).one()


class Rate(db.Model, LoadSingleton):
    """
    Model to store rate value
    """

    __tablename__ = "rate"

    id = db.Column(db.Integer, primary_key=True)
    value = db.Column(db.Float, nullable=False, default=500)
    
    def __repr__(self):
        return f"<Rate value:{self.value}>"

# Usage
rate = Rate().load().value