Create a private attribute for the Singleton Pattern called _instance.
为Singleton模式创建私有属性，称为_ instance。
The self.__last instance attribute (which is a private attribute because it starts with a double underscore) holds the previous mouse click position.
__last实例特性(它是一个私有特性，因为它以双下划线开头)包含上一次鼠标单击位置。
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