Sandra-24 wrote:
> Now that is a clever little trick. I never would have guessed you can
> assign to __class__, Python always surprises me in it's sheer
> flexibility.
>
> In this case it doesn't work.
>
> TypeError: __class__ assignment: only for heap types
>
> I suspect that's because this object begins its life in C code.
>
> The technique of using the __class__.__subclasses__ also fails:
>
> TypeError: cannot create 'B' instances
>
> This seems more complex than I thought. Can one do this for an object
> that beings it's life in C?
The restriction originates in the metaclass. Perhaps you can use a
customized metaclass that allows subclass assignment, but I don't know
enough about Python's internals to tell you how/whether that is possible.
An alternative might be that you always start out with a subclass coded in
Python:
>>> abc = tuple("abc")
>>> abc.__class__ = tuple
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: __class__ assignment: only for heap types
>>> class Tuple(tuple): pass
...
>>> class Subclass(Tuple):
... first = property(lambda self: self[0])
...
>>> abc = Tuple("abc")
>>> abc.__class__ = Subclass
>>> abc.first
'a'
In the example a Tuple should be able to do everything a tuple can do;
because Tuple is coded in Python you can also change the __class__.
Peter
--
http://mail.python.org/mailman/listinfo/python-list
