Python中根据属性值获取现有对象实例：Metaclass与对象唯一性管理

class Tree:
    def __init__(self, name, cell=""):
        self.name = name
        self.cell = cell
        self.children = []
        self.parent = None

    def add_child(self, child):
        child.parent = self
        self.children.append(child)

# 示例操作
node = Tree("A", cell="A_cell")
node.add_child(Tree("B", cell="B_cell"))
node.add_child(Tree("C", cell="C_cell"))

# 尝试通过名称获取对象
print(Tree("B").cell)

import weakref

class MetaTree(type):
    # 使用WeakValueDictionary存储实例，name作为键
    instances = weakref.WeakValueDictionary()

    def __call__(cls, name, cell=""):
        # 尝试从instances中获取指定name的实例
        if not (instance := cls.instances.get(name)):
            # 如果不存在，则创建新实例
            instance = cls.__new__(cls) # 调用类的__new__方法创建原始对象
            instance.__init__(name, cell) # 调用类的__init__方法初始化对象
            cls.instances[name] = instance # 将新实例存储起来
        return instance # 返回现有或新创建的实例

class Tree(metaclass=MetaTree):
    def __init__(self, name, cell=""):
        self.name = name
        self.cell = cell
        self.children = []
        self.parent = None

    def add_child(self, child):
        child.parent = self
        self.children.append(child)

# 改进后的示例操作
node = Tree("A", cell="A_cell")
node.add_child(Tree("B", cell="B_cell"))
node.add_child(Tree("C", cell="C_cell"))
node.add_child(Tree("D", cell="D_cell"))

# 再次尝试通过名称获取对象
print(Tree("B").cell)

node_b = Tree("B", cell="B_cell")
node_b.name = "X" # 修改了name属性
print(Tree("B").cell) # 仍然会返回node_b，但其name已是"X"，语义混乱
print(Tree("X").cell) # 可能会创建新的"X"对象，或者如果"X"已存在，返回另一个对象

import weakref

class MetaTree(type):
    instances = weakref.WeakValueDictionary()
    def __call__(cls, name, cell=""):
        if not (instance := cls.instances.get(name)):
            instance = cls.__new__(cls)
            instance.__init__(name, cell)
            cls.instances[name] = instance
        return instance

class Tree(metaclass=MetaTree):
    def __init__(self, name, cell=""):
        self._name = name  # 存储在私有变量中
        self.cell = cell
        self.children = []
        self.parent = None

    @property
    def name(self):
        """提供只读的name属性"""
        return self._name

    def add_child(self, child):
        child.parent = self
        self.children.append(child)

# 示例操作
node_a = Tree("A", cell="A_cell")
node_b = Tree("B", cell="B_cell")
node_a.add_child(node_b)

print(Tree("B").cell) # 输出: B_cell

# 尝试修改name属性，这会引发AttributeError
try:
    node_b.name = "X"
except AttributeError as e:
    print(f"尝试修改只读属性失败: {e}")

print(Tree("B").name) # 仍然是B