object vs class oriented -- xotcl

[Steven D'Aprano]

On Thu, 24 Jan 2008 12:35:44 -0800, William Pursell wrote:

> Basically, you can
> instantiate an object A of class Foo, and later change A to be an object
> of class Bar.   Does Python support this type of flexibility? As I
> stated above, I've been away from Python for awhile now, and am a bit
> rusty,  but it seems that slots or "new style" objects might provide
> this type of behavior.

If you think slots are a candidate for this functionality, I think you 
have misunderstood what slots actually are. Slots don't define what class 
the object has, they are a memory optimization for when you need vast 
numbers of instances and don't need arbitrary attributes.


> The ability to have an object change class is
> certainly  (to me) a novel idea.  Can I do it in Python?

Yes, mostly. Example:

>>> class Spam(object):
...     def whatami(self):
...             return "I am a delicious and tasty processed meat product"
...
>>> class Parrot(object):
...     def whatami(self):
...             return "I am a colourful bird with a large vocabulary"
...
>>>
>>> s = Spam()
>>> s.whatami()
'I am a delicious and tasty processed meat product'
>>> s.__class__ = Parrot
>>> s.whatami()
'I am a colourful bird with a large vocabulary'


If you actually play around with this, you'll soon find the limitations. 
For instance:

>>> s.__class__ = int
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __class__ assignment: only for heap types




-- 
Steven