什么是AttributeSet以及如何使用它？

class Attribute:
    def __init__(self, name, value):
        self.name = name
        self.value = value

class AttributeSet:
    def __init__(self):
        self.attributes = {}

    def add(self, attribute):
        self.attributes[attribute.name] = attribute.value

    def remove(self, name):
        if name in self.attributes:
            del self.attributes[name]

    def update(self, attribute):
        if attribute.name in self.attributes:
            self.attributes[attribute.name] = attribute.value

    def get(self, name):
        if name in self.attributes:
            return self.attributes[name]
        else:
            return None