gistfile1.py

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.sql.functions import coalesce
from sqlalchemy.orm import sessionmaker
from sqlalchemy import Column, Integer, String, select
from sqlalchemy import create_engine
from sqlalchemy import func

engine = create_engine('sqlite:///:memory:', echo=True)
Base = declarative_base()


class Foo(Base):
    __tablename__ = "foo"
    id = Column(Integer, primary_key=True)
    num = Column(Integer)


Session = sessionmaker(bind=engine)
session = Session()
Base.metadata.create_all(engine)


foo = Foo()
foo.num = select([coalesce(func.max(Foo.num), 0) + 1]).where(Foo.id > 0)
session.add(foo)
session.commit()

foos = session.query(Foo).all()
print foos[0].num