SqlAlchemy usage with tornado backend

backend.py

class Backend(object):
  def __init__(self):
    engine = create_engine("mysql://{0}:{1}@{2}/{3}".format(options.mysql_user, options.mysql_pass, options.mysql_host, options.mysql_db)
                            , pool_size = options.mysql_poolsize
                            , pool_recycle = 3600
                            , echo=options.debug
                            , echo_pool=options.debug)
    self._session = sessionmaker(bind=engine)
  
  @classmethod
  def instance(cls):
    """Singleton like accessor to instantiate backend object"""
    if not hasattr(cls,"_instance"):
      cls._instance = cls()
    return cls._instance
  
  def get_session(self):
    return self._session()

base_handler.py

class BaseHandler(tornado.web.RequestHandler):
  @property
  def backend(self):
    return Backend.instance()

handler.py

class ExampleHandler(BaseHandler):
  def get(self):
    session = self.backend.get_session()
    try:
      """Everything I need to do here"""
      session.commit()
    except Exception as e:
      session.rollback()
    finally:
      session.close()