Convert A BaseClass Object Into A SubClass Object Idiomatically?

There is a base class Base and a subclass Special. class Base(object): def __init__(self, name): self.name = name def greet(self): return 'Hello %s' % self.

Solution 1:

You can achieve this by defining an alternate constructor and reassigning the instance's __class__ attribute.

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?

EDIT:

Moved the constructor from Special to Base.

If you can't modify the Base class you can add a classmethod to Special that will change the class of any object passed to it.

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'>

A more all purpose solution would be to create a mixin that can be added to any class.

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'>

Solution 2:

actually you can, but I don't think you should. instead of typecasting python has duck-typing to solve this kind of situation.

anyway, here's the code:

>>> 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?"