如何在flask-SQLAlchemy中使用Default和Onupdate之类的值

My Code,

class User(db.Model, UserMixin):
  uid = db.Column(db.Integer, primary_key=True)
  username = db.Column(db.String(250), unique=True)
  password = db.Column(db.String(250))
  name = db.Column(db.String(250))
  created_on = db.Column(db.TIMESTAMP, default=False)
  modified_on = db.Column(db.TIMESTAMP, onupdate=datetime.datetime.now)
class User(db.Model, UserMixin):
  u