我总是犯同样的错误。我已经安装了所有我能安装的，比如：'pip install flask\u sqlalchemy'和'pip install sqlalchemy'等等，但是它仍然给我同样的错误。
我安装的软件包
我的代码：`

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__) 
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:////tmp/test.db' # load the configuration of choice
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False
db = SQLAlchemy(app)

class Books(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(120), unique=True, nullable=False)
    author = db.Column(db.String(120), unique=True, nullable=False)
    rating = db.Column(db.Float(10), unique=True, nullable=False)

db.create_all()

new_book = Books(id=1, title="Harry Potter", author="J. K. Rowling", rating=9.3)
db.session.add(new_book)
db.session.commit()`