使用嵌套的弱引用创建类实例的深层副本

Question

我有两个类：一个父类和一个容器类。父类实例有匹配的容器类实例作为弱引用。

深拷贝父实例时出现问题，weakref 仍然链接到原始实例。这是一个最小的例子：

import weakref
from copy import deepcopy


class Container:    
    def __init__(self, parent):
        self.parent = weakref.ref(parent)


class Parent:
    def __init__(self):
        self.container = Container(self)


if __name__ == '__main__':
    parent1 = Parent()
    assert(parent1 is parent1.container.parent())

    parent2 = deepcopy(parent1)
    assert(parent2 is parent2.container.parent())

第二个断言失败。

我怀疑__deepcopy__这个魔术方法可以实现，但不确定如何实现。

Answer 1

问题是deepcopy 不会跟随weakref.ref 链接。它甚至没有复制weakref.ref:

>>> from copy import deepcopy
>>> import weakref
>>>
>>> parent1 = Parent()
>>> ref1 = weakref.ref(parent1)
>>> ref2 = deepcopy(ref1)
>>> ref1 is ref2
True

这是明确硬编码的in the copy module。我不知道这是为什么，但我怀疑他们有他们的原因。

但是您可以实现__deepcopy__ 方法：

import weakref
from copy import deepcopy

class Container:    
    def __init__(self, parent):
        self.parent = weakref.ref(parent)

class Parent:
    def __init__(self):
        self.container = Container(self)

    def __deepcopy__(self, memo):
        # set __deepcopy__ element to "false"-ey value so we don't go into
        # recusion.
        self.__deepcopy__ = None
        try:
            new = deepcopy(self, memo)
        finally:
            # Always delete the self.__deepcopy__ again, even if deepcopying failed
            del self.__deepcopy__
        del new.__deepcopy__  # remove the copied __deepcopy__ attribute
        new.container.parent = weakref.ref(new)
        return new

if __name__ == '__main__':
    parent1 = Parent()
    assert parent1 is parent1.container.parent()

    parent2 = deepcopy(parent1)
    assert parent2 is parent2.container.parent()

    parent3 = deepcopy(parent2)
    assert parent3 is parent3.container.parent()

由于临时的__deepcopy__ 实例属性，它有点难看。但它允许在 self 上使用普通的 deepcopy 函数，而无需进入无限递归，然后您只需手动创建对父级的新弱引用。

您甚至可以在不临时设置__deepcopy__ 的情况下执行此操作（它应该可以正常工作）：

import weakref
from copy import deepcopy

class Container:    
    def __init__(self, parent):
        self.parent = weakref.ref(parent)

class Parent:
    def __init__(self):
        self.container = Container(self)

    def __deepcopy__(self, memo):
        # Create a new class
        new = object.__new__(type(self))
        memo[id(self)] = new   # add the new class to the memo
        # Insert a deepcopy of all instance attributes
        new.__dict__.update(deepcopy(self.__dict__, memo))
        # Manually update the weakref to be correct
        new.container.parent = weakref.ref(new)
        return new

if __name__ == '__main__':
    parent1 = Parent()
    assert parent1 is parent1.container.parent()

    parent2 = deepcopy(parent1)
    assert parent2 is parent2.container.parent()

    parent3 = deepcopy(parent2)
    assert parent3 is parent3.container.parent()