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使用python deepcopy时的AttributeError

cyclic-reference deep-copy python

Evgeny Tanhilevich · 技术社区 · 8 年前

__eq__ 和 __hash__ 重写,使其对象充当字典键。每个对象还携带一个字典,由同一类的其他对象键入。我有一个奇怪的 AttributeError 当我试着 deepcopy 整个结构。我在OsX上使用Python 3.6.0。

Python docs 深度复制 使用 memo 字典来缓存它已经复制的对象,因此嵌套结构不应该是一个问题。那么我做错了什么?我应该自己编码吗 __deepcopy__

from copy import deepcopy


class Node:

    def __init__(self, p_id):
        self.id = p_id
        self.edge_dict = {}
        self.degree = 0

    def __eq__(self, other):
        return self.id == other.id

    def __hash__(self):
        return hash(self.id)

    def add_edge(self, p_node, p_data):
        if p_node not in self.edge_dict:
            self.edge_dict[p_node] = p_data
            self.degree += 1
            return True
        else:
            return False

if __name__ == '__main__':
    node1 = Node(1)
    node2 = Node(2)
    node1.add_edge(node2, "1->2")
    node2.add_edge(node1, "2->1")
    node1_copy = deepcopy(node1)

File ".../node_test.py", line 15, in __hash__
    return hash(self.id)
AttributeError: 'Node' object has no attribute 'id'

1 回复 | 直到 7 年前

ShadowRanger 8 年前

循环依赖性对于 deepcopy 当你:

建设 ,而不仅仅是 初始化

问题是取消勾选对象( 深度复制 默认情况下,通过pickle和unpickle复制自定义对象,除非特殊 __deepcopy__ 方法)在不初始化的情况下创建空对象,然后尝试逐个填充其属性。当它试图填补 node1 的属性,它需要初始化 node2 节点1 (在这两种情况下,由于 edge_dict 边缘dict 一个人 Node 节点 它增加了没有它的 id 属性尚未设置,因此对其进行哈希的尝试失败。

__new__ 确保在初始化可变(可能是递归)属性和定义 pickle 帮手 __getnewargs__ (或 __getnewargs_ex__ )使其正确使用。具体来说,将类定义更改为:

class Node:
    # __new__ instead of __init__ to establish necessary id invariant
    # You could use both __new__ and __init__, but that's usually more complicated
    # than you really need
    def __new__(cls, p_id):
        self = super().__new__(cls)  # Must explicitly create the new object
        # Aside from explicit construction and return, rest of __new__
        # is same as __init__
        self.id = p_id
        self.edge_dict = {}
        self.degree = 0
        return self  # __new__ returns the new object

    def __getnewargs__(self):
        # Return the arguments that *must* be passed to __new__
        return (self.id,)

    # ... rest of class is unchanged ...

object 和改变 super() super(Node, cls) 在里面 __新建__ ; 给出的代码是更简单的Python 3代码。

只有 copy.deepcopy __新建__ / (需要新样式的类)将仅覆盖深度复制。您需要在原始类上定义以下额外方法(并确保模块导入 copy ),否则将其保持不变:

def __deepcopy__(self, memo):
    # Deepcopy only the id attribute, then construct the new instance and map
    # the id() of the existing copy to the new instance in the memo dictionary
    memo[id(self)] = newself = self.__class__(copy.deepcopy(self.id, memo))
    # Now that memo is populated with a hashable instance, copy the other attributes:
    newself.degree = copy.deepcopy(self.degree, memo)
    # Safe to deepcopy edge_dict now, because backreferences to self will
    # be remapped to newself automatically
    newself.edge_dict = copy.deepcopy(self.edge_dict, memo)
    return newself