python console 内from app import db报错

python console 内from app  import db为什么会报一下错误

from app import db
Traceback (most recent call last):
  File "<input>", line 1, in <module>
ImportError: cannot import name 'db' from 'app' (D:\Python\WEB\app.py)

程序代码如下：

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SECRET_KEY'] = 'lol'
# 配置数据库的连接参数
app.config['SQLALCHEMY_DATABASE_URI']= 'mysql://root:123456@127.0.0.1/test'
db = SQLAlchemy(app)


class User(db.Model):
    __tablename__ = 'weibo_user'
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(64), nullable=False)
    password = db.Column(db.String(256), nullable=False)
    birth_date = db.Column(db.Date, nullable=True)
    age = db.Column(db.Integer, default=0)