Type casting a base class to a derived one?

[Paul McGuire]

"Peter Otten" <__peter__ at web.de> wrote in message 
news:eo5bia$re6$00$1 at news.t-online.com...
> Frederic Rentsch wrote:
>
>>    If I derive a class from another one because I need a few extra
>> features, is there a way to promote the base class to the derived one
>> without having to make copies of all attributes?
>>
>> class Derived (Base):
>>    def __init__ (self, base_object):
>>       # ( copy all attributes )
>>       ...
>>
>> This looks expensive. Moreover __init__ () may not be available if it
>> needs to to something else.
>
> base_instance = Base(...)
> base_instance.__class__ = Derived
>
> Peter

This will change the class-level behavior of the object, but it wont 
automagically populate the instance with any attributes that are 
specifically added in Derived's __init__.

class A(object):
    def __init__(self,x):
        self.x = x
    def method(self):
        print "I am an A whose x value is", self.x

class B(A):
    bb = 1000
    def __init__(self,x,y):
        super(B,self).__init__(x)
        self.y = y
    def method(self):
        print "I am a B whose x value is", self.x

aobj = A(100)
aobj.method()
aobj.__class__ = B
aobj.method()
print aobj.bb
print aobj.y

prints:
I am an A whose x value is 100
I am a B whose x value is 100
1000
Traceback (most recent call last):
  File "dertest.py", line 20, in ?
    print aobj.y
AttributeError: 'B' object has no attribute 'y'


But it wouldn't be hard to write a makeBaseIntoDerived method:

def makeAintoB(a,y=0):
    a.y = y
    a.__class__ = B

-- Paul