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的实例转换为实例Special？目前我在classmethod上定义了Special，只需重新__dict__：

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

这是惯用的吗？如果不是会是什么？

P.S。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。现在在某个项目中，基类已经是子类，并且已经在子类中添加了特殊方法。现在你仍然主要使用基类对象，但有时你会想要“升级”到特殊类，因为你需要访问一些方法。

Answer 1

您可以通过定义备用构造函数并重新分配实例的__class__属性来实现此目的。

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?

修改

将构造函数从Special移至Base。

如果您无法修改Base类，可以向Special添加一个类方法，该类方法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'>

更全能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'>

Answer 2

实际上你可以，但我认为你不应该。而不是类型转换python有鸭子打字来解决这种情况。

无论如何，这是代码：

>>> base = Base("I'm a base!")
>>> hasattr(base, 'rhyme')
False
>>> base.__class__ = Special
>>> hasattr(base, 'rhyme')
True
>>> base.rhyme()
"Hi I'm a base!! How are you? Fine, thanks. What about you?"

是否以惯用方式将BaseClass对象转换为SubClass对象？

2 个答案: