当我注册时，我的数据库没有添加用户

from sqlalchemy import create_engine, Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

Base = declarative_base()

class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
    username = Column(String)
    email = Column(String)

engine = create_engine('sqlite:///example.db')
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session = Session()

new_user = User(username='john_doe', email='john@example.com')
session.add(new_user)
session.commit()

from sqlalchemy import create_engine

engine = create_engine('sqlite:///example.db')
connection = engine.connect()
connection.close()

DESCRIBE users;