# 单例装饰器
class singleton:
    """
    单例装饰器，确保类只有一个实例
    """
    def __init__(self, cls):
        self.cls = cls
        self._instance = None
    
    def __call__(self, *args, **kwargs):
        if self._instance is None:
            self._instance = self.cls(*args, **kwargs)
        return self._instance