在加载旧对象时使用 python 的属性

Question

我有一个相当大的项目，包括一个 class Foo，它最近需要使用 @property 装饰器进行更新以创建自定义 getter 和 setter 方法。

我还在我的硬盘上存储了几个 Foo 的实例，在某些时候我可能需要重新加载。我的问题是，我无法访问这些旧对象上用属性修饰的属性。

考虑以下示例：

import pickle

# define Class and create instance
class Foo:
    def __init__(self):
        self.val = 1
foo = Foo()

# dump foo into file
with open("foo.pickle", 'wb') as handle:
    pickle.dump(foo, handle, pickle.HIGHEST_PROTOCOL)

# overwrite and add @property in the class definition
class Foo:
    def __init__(self):
        self._val = "new_foo"

    @property
    def val(self):
        return self._val

    @val.setter
    def val(self, val):
        self._val = val

foo_new = Foo()
print(foo_new.val)

# reload foo
with open("foo.pickle", "rb") as handle:
    foo_old = pickle.load(handle)

# try to access attributes
print(foo_old.val)

最后一行加注：

NameError: name '_val' is not defined

我还有哪些选项可以访问我的存档实例的属性？

编辑：在第二个 Foo 定义的构造函数中将 self.val 更改为 self._val。

Answer 1

这可能完全是黑客攻击 -- 我不确定。但是，我能够使用以下代码重建从您的第一个 "Foo" class 中腌制的对象；

import pickle

class Foo:
    def __init__(self):
        self._val = "new_foo"

    @property
    def val(self):
        try:
            return self._val
        except AttributeError:
            self._val = self.__dict__['val']
            self.__dict__.pop('val')
            return self._val

    @val.setter
    def val(self, val):
        self._val = val


with open("foo.pickle", "rb") as handle:
    foo_old = pickle.load(handle)

print(foo_old.val)

Answer 2

一种可能性是使用自定义 Unpickler，尽管您需要保留旧的 class（使用不同的名称，如果需要则隐藏）并定义转换逻辑旧对象 class 到新对象。这是一个基本示例：

import pickle

# define Class and create instance
class Foo:
    def __init__(self):
        self.val = 1
foo = Foo()

# dump foo into file
with open("foo.pickle", 'wb') as handle:
    pickle.dump(foo, handle, pickle.HIGHEST_PROTOCOL)

# Old class is kept with a new name
FooOld = Foo

# overwrite and add @property in the class definition
class Foo:
    def __init__(self):
        self._val = "new_foo"

    @property
    def val(self):
        return self._val

    @val.setter
    def val(self, val):
        self._val = val

foo_new = Foo()
print(foo_new.val)

# Custom Unpickler
class FooOldUnpickler(pickle.Unpickler):
    def __init__(self, *args, **kwargs):
        super(FooOldUnpickler, self).__init__(*args, **kwargs)

    def load(self):
        obj = super(FooOldUnpickler, self).load()
        if type(obj) is FooOld:
            # Object conversion logic
            newObj = Foo()
            newObj.val = obj.val
            obj = newObj
        return obj

    def find_class(self, module, name):
        # Use old class instead of new for loaded objects
        if module == __name__ and name == 'Foo':
            return FooOld
        return super(FooOldUnpickler, self).find_class(module, name)

# reload foo
with open("foo.pickle", "rb") as handle:
    # Use custom unpickler
    foo_old = FooOldUnpickler(handle).load()

# try to access attributes
print(foo_old.val)

Answer 3

泡菜documentation说：

When a class instance is unpickled, its __init__() method is usually not invoked.

这就是未定义 _val 属性的原因您可以通过在替换 Foo class 中定义 __new__ 方法并设置实例属性来解决此问题那里：

import pickle

# define Class and create instance
class Foo:
    def __init__(self):
        self.val = 1
foo = Foo()

# dump foo into file
with open("foo.pickle", 'wb') as handle:
    pickle.dump(foo, handle, pickle.HIGHEST_PROTOCOL)

# overwrite and add @property in the class definition
class Foo:
    def __new__(cls, val=None):
        inst = super().__new__(cls)
        inst._val = "new_foo"  if val is None else val
        return inst

    @property
    def val(self):
        return self._val

    @val.setter
    def val(self, val):
        self._val = val

foo_new = Foo()
print(foo_new.val)  # -> new_foo

# reload foo
with open("foo.pickle", "rb") as handle:
    foo_old = pickle.load(handle)

print(foo_old.val)  # -> new_foo

在加载旧对象时使用 python 的属性

Using python's property while loading old objects

python

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在加载旧对象时使用 python 的 属性

Using python's property while loading old objects

python

properties

pickle

在加载旧对象时使用 python 的属性