如何通过App Engine使用云SQL选择要查询的数据库？

runtime: python39
env_variables:
  CLOUD_SQL_CONNECTION_NAME: <云SQL实例连接名>
  DB_USER: <数据库用户名>
  DB_PASSWORD: <数据库密码>

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'mysql+mysqldb://${DB_USER}:${DB_PASSWORD}@/${CLOUD_SQL_CONNECTION_NAME}?unix_socket=/cloudsql/${CLOUD_SQL_CONNECTION_NAME}'
db = SQLAlchemy(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))

@app.route('/')
def index():
    # 查询数据库
    users = User.query.all()
    return 'Hello, {}'.format(users[0].name)

if __name__ == '__main__':
    app.run()