Python Flask上有没有单例模式？

class Singleton:
    _instance = None

    def __new__(cls, *args, **kwargs):
        if not cls._instance:
            cls._instance = super().__new__(cls, *args, **kwargs)
        return cls._instance

from flask import Flask
from singleton import Singleton

app = Flask(__name__)
db = Singleton()  # 使用单例模式创建数据库连接实例

@app.route('/')
def index():
    # 使用单例模式中的数据库连接实例进行数据库操作
    db.query('SELECT * FROM table')
    return 'Hello World'

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