我怎样才能在python中挑选一个嵌套类？

Question

我有一个嵌套类:

class WidgetType(object):

    class FloatType(object):
        pass

    class TextType(object):
        pass

..和一个引用嵌套类类型(不是它的实例)的对象,就像这样

class ObjectToPickle(object):
     def __init__(self):
         self.type = WidgetType.TextType

尝试序列化ObjectToPickle类的实例会导致:

PicklingError:无法挑选<class'setmanager.app.site.widget_data_types.TextType'>

有没有办法在python中腌制嵌套类？

Answer 1

pickle模块试图从模块中获取TextType类.但由于该类是嵌套的,因此不起作用.jasonjs的建议会奏效.以下是pickle.py中负责错误消息的行:

    try:
        __import__(module)
        mod = sys.modules[module]
        klass = getattr(mod, name)
    except (ImportError, KeyError, AttributeError):
        raise PicklingError(
            "Can't pickle %r: it's not found as %s.%s" %
            (obj, module, name))

klass = getattr(mod, name)当然不会在嵌套类的情况下工作.为了演示正在发生的事情,尝试在pickle实例之前添加这些行:

import sys
setattr(sys.modules[__name__], 'TextType', WidgetType.TextType)

此代码将TextType作为属性添加到模块中.酸洗应该工作得很好.我不建议你使用这个黑客.

Answer 2

我知道这是一个非常古老的问题,但我从未明确地看到过这个问题的令人满意的解决方案,除了重新构造代码的明显且最可能正确的答案.

不幸的是,它并不总是可行的做这样的事情,在这种情况下,作为最后的手段,它是可能的泡菜被另一个类中定义的类的实例.

该__reduce__函数的python文档说明您可以返回

将调用的可调用对象,用于创建对象的初始版本.元组的下一个元素将为此可调用对象提供参数.

因此,您只需要一个可以返回相应类的实例的对象.这个类本身必须是可选择的(因此,必须存在于__main__关卡中),并且可以简单如下:

class _NestedClassGetter(object):
    """
    When called with the containing class as the first argument, 
    and the name of the nested class as the second argument,
    returns an instance of the nested class.
    """
    def __call__(self, containing_class, class_name):
        nested_class = getattr(containing_class, class_name)
        # return an instance of a nested_class. Some more intelligence could be
        # applied for class construction if necessary.
        return nested_class()

因此,剩下的就是__reduce__在FloatType上的方法中返回适当的参数:

class WidgetType(object):

    class FloatType(object):
        def __reduce__(self):
            # return a class which can return this class when called with the 
            # appropriate tuple of arguments
            return (_NestedClassGetter(), (WidgetType, self.__class__.__name__, ))

结果是一个嵌套的类,但实例可以被pickle(需要进一步的工作来转储/加载__state__信息,但根据__reduce__文档,这是相对简单的).

这种相同的技术(轻微的代码修改)可以应用于深层嵌套的类.

一个完整的例子:

import pickle


class ParentClass(object):

    class NestedClass(object):
        def __init__(self, var1):
            self.var1 = var1

        def __reduce__(self):
            state = self.__dict__.copy()
            return (_NestedClassGetter(), 
                    (ParentClass, self.__class__.__name__, ), 
                    state,
                    )


class _NestedClassGetter(object):
    """
    When called with the containing class as the first argument, 
    and the name of the nested class as the second argument,
    returns an instance of the nested class.
    """
    def __call__(self, containing_class, class_name):
        nested_class = getattr(containing_class, class_name)

        # make an instance of a simple object (this one will do), for which we can change the
        # __class__ later on.
        nested_instance = _NestedClassGetter()

        # set the class of the instance, the __init__ will never be called on the class
        # but the original state will be set later on by pickle.
        nested_instance.__class__ = nested_class
        return nested_instance



if __name__ == '__main__':

    orig = ParentClass.NestedClass(var1=['hello', 'world'])

    pickle.dump(orig, open('simple.pickle', 'w'))

    pickled = pickle.load(open('simple.pickle', 'r'))

    print type(pickled)
    print pickled.var1

我对此的最后一点是要记住其他答案所说的内容:

如果您有能力这样做,请考虑重新分解代码以避免首先使用嵌套类.

Answer 3

在Sage(www.sagemath.org)中,我们有很多这种酸洗问题.我们决定系统地解决它的方法是将外部类放在特定的元类中,其目标是实现和隐藏黑客.请注意,如果存在多个嵌套级别,则会自动通过嵌套类传播.

Answer 4

如果你使用dill而不是pickle,它的工作原理.

>>> import dill
>>> 
>>> class WidgetType(object):
...   class FloatType(object):
...     pass
...   class TextType(object):
...     pass
... 
>>> class ObjectToPickle(object):
...   def __init__(self):
...     self.type = WidgetType.TextType
... 
>>> x = ObjectToPickle()
>>> 
>>> _x = dill.dumps(x)
>>> x_ = dill.loads(_x)
>>> x_
<__main__.ObjectToPickle object at 0x10b20a250>
>>> x_.type
<class '__main__.TextType'>

在这里获取莳萝:https://github.com/uqfoundation/dill

Answer 5

Pickle 仅适用于模块范围（顶层）中定义的类。在这种情况下，看起来您可以在模块范围中定义嵌套类，然后将它们设置为 WidgetType 上的属性，假设有理由不只在代码中引用TextTypeand 。FloatType或者，导入它们所在的模块并使用widget_type.TextType和widget_type.FloatType。