创建具有嵌套的weakref的类实例的深层副本

问题描述

我有两个类：父类和容器类。父类实例具有匹配的容器类实例作为弱引用。

I have two classes: a parent class and a container class. The parent class instance has matching container class instance as a weak reference.

深度复制父实例时出现问题，weakref仍链接到原始实例。这是一个最小的示例：

There is a problem while deep copying the parent instance, the weakref is still linking to the original instance. Here is a minimal example:

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 __ 魔术方法可以实现，但不确定具体如何。

I suspect that the __deepcopy__ magic method can be implemented, but not sure how exactly.

推荐答案

问题是 deepcopy 将不会跟随 weakref.ref 链接。它甚至不会复制 weakref.ref ：

The problem is that deepcopy won't follow the weakref.ref link. It doesn't even copy the weakref.ref:

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

即已明确硬编码 。我不知道为什么会这样，但是我怀疑他们有他们的原因。

That is explicitly hardcoded in the copy module. I don't know why this is but I suspect they had their reasons.

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，这有点难看。但是它允许在 self 上使用普通的 deepcopy 函数，而无需进行无限递归，那么您只需要手动创建

It's a bit ugly because of the temporary __deepcopy__ instance attribute. But it allows to use the normal deepcopy function on self without going into infinite recursions and then you only have to manually create a new weak reference to your parent.

您甚至可以在不临时设置 __ deepcopy __ 的情况下进行操作（它应该可以正常工作）：

You could even do it without temporarily setting __deepcopy__ (it should work correctly):

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()

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