This is from my blog post on pynash.org on 2013-03-01

2013-03-01-SQLAlchemy-Hybrid-Properties.py

from sqlalchemy import create_engine
engine = create_engine('sqlite:///:memory', echo=True)
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.hybrid import hybrid_property
Base = declarative_base()
from sqlalchemy import Column, Integer, String, Float
class Order(Base):
    __tablename__ = 'orders'
    order_id = Column(Integer, primary_key=True)
    order_reference_code = Column(String)
    merchandise_cost = Column(Float)
    tax = Column(Float)
    shipping = Column(Float)
    def __init__(self, order_reference_code, merchandise_cost, tax, shipping):
        self.order_reference_code = order_reference_code
        self.merchandise_cost = merchandise_cost
        self.tax = tax
        self.shipping = shipping
    @hybrid_property
    def grand_total(self):
        rollup_fields = [
        'merchandise_cost',
        'tax',
        'shipping',
        ]
        total = sum([self.__getattribute__(x) for x in rollup_fields])
        return round(total, 2)

Base.metadata.create_all(engine)
    
order1 = Order('ABC', 57.50, 11.75, 6.00)
order2 = Order('BCD', 21.42, 4.72, 3.00)
order3 = Order('CDE', 1000.50, 80.34, 0.00)
order4 = Order('DEF', 500.50, 45.34, 100.00)

from sqlalchemy.orm import sessionmaker
Session = sessionmaker(bind=engine)
db = Session()

db.add(order1)
db.add(order1)
db.add(order1)
db.add(order1)
db.commit()

for order in db.query(Order).all():
    print order.order_reference_code, order.grand_total