锁定sqlalchemy

我对如何同时修改来自多个不同进程的表感到困惑。我试过用 Query.with_lockmode()

import time
from sqlalchemy.orm import sessionmaker, scoped_session
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import *

engine = create_engine('mysql://...?charset=utf8&use_unicode=0', pool_recycle=3600, echo=False)
Base = declarative_base(bind=engine)
session = scoped_session(sessionmaker(engine))

class Test(Base):
    __tablename__ = "TESTXYZ"
    id = Column(Integer, primary_key=True)
    x = Column(Integer)

def keepUpdating():
    test = session.query(Test).filter(Test.id==1).with_lockmode("update").one()

    for counter in range(5):
        test.x += 10
        print test.x
        time.sleep(2)

    session.commit()


keepUpdating()

session.query(Test).filter(Test.id==1).one().x 等于50,而不是100(假设一开始是0),这正是我所希望的。如何让两个进程同时更新值,或者让第二个进程等待第一个进程完成?