python - 惯用地将 BaseClass 对象转换为 SubClass 对象？

Question

有一个基类Base和一个子类Special。

class Base(object):
    def __init__(self, name):
        self.name = name
    def greet(self):
        return 'Hello %s' % self.name

class Special(Base):
    def __init__(self, name):
        super(Special, self).__init__(name)
    def rhyme(self):
        return 'Hi %s! How are you? Fine, thanks. What about you?' % self.name

如何将 Base 实例转换为 Special 实例？目前，我在 Special 上定义了一个 classmethod，它只是重新分配了 __dict__:

class Special(Base):
    ...
    @classmethod
    def from_base(cls, baseobj):
        special = Special()
        special.__dict__ = baseobj.__dict__
        return special

这是惯用语吗？如果不是会是什么？

附言示例场景:基类是一些默认实现。在野外，您可能会找到基类的对象。现在在一些项目中，基类已经是子类，并且在子类中添加了特殊的方法。现在你大部分时间仍然使用基类对象，但有时你会想“升级”到特殊类，因为你需要访问一些方法。

Answer 1

您可以通过定义替代构造函数并重新分配实例的 __class__ 属性来实现这一点。

class Base(object):
    def __init__(self, name):
        self.name = name

    def greet(self):
        return 'Hello %s' % self.name

    @classmethod
    def alt_constructor(cls, *args, **kwargs):
        obj = cls(*args, **kwargs)
        obj.__class__ = Special
        return obj


class Special(Base):
    def __init__(self, name):
        super(Special, self).__init__(name)

    def rhyme(self):
        return 'Hi %s! How are you? Fine, thanks. What about you?' % self.name


>>> s = Base.alt_constructor("test")
>>> print s.rhyme()
Hi test! How are you? Fine, thanks. What about you?

编辑:

将构造函数从 Special 移至 Base。

如果您不能修改 Base 类，您可以向 Special 添加一个类方法，这将更改传递给它的任何对象的类。

class Base(object):
    def __init__(self, name):
        self.name = name

    def greet(self):
        return 'Hello %s' % self.name


class Special(Base):
    def __init__(self, name):
        super(Special, self).__init__(name)

    def rhyme(self):
        return 'Hi %s! How are you? Fine, thanks. What about you?' % self.name

    @classmethod
    def convert_to_special(cls, obj):
        obj.__class__ = Special

>>> b = Base("test")
>>> print type(b)
<class '__main__.Base'>

>>> Special.convert_to_special(b)
>>> print type(b)
<class '__main__.Special'>

一个更通用的解决方案是创建一个可以添加到任何类的 mixin。

class ConverterMixin(object):

    @classmethod
    def convert_to_class(cls, obj):
        obj.__class__ = cls


class Special(ConverterMixin, Base):
    def __init__(self, name):
        super(Special, self).__init__(name)

    def rhyme(self):
        return 'Hi %s! How are you? Fine, thanks. What about you?' % self.name

>>> b = Base("test")
>>> print type(b)
<class '__main__.Base'>

>>> Special.convert_to_class(b)
>>> print type(b)
<class '__main__.Special'>