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?
Thanks,
-Sandra
Peter Otten wrote:
> Sandra-24 wrote:
>
> > Can you create an instance of a subclass using an existing instance of
> > the base class?
> >
> > Such things would be impossible in some languages or very difficult in
> > others. I wonder if this can be done in python, without copying the
> > base class instance, which in my case is a very expensive object.
>
> You can change the class of an instance by assigning to the __class__
> attribute. The new class doesn't even need to be a subclass of the old:
>
> >>> class A(object):
> ... def __init__(self, name):
> ... self.name = name
> ... def show(self): print self.name
> ...
> >>> a = A("alpha")
> >>> a.show()
> alpha
> >>> class B(object):
> ... def show(self): print self.name.upper()
> ...
> >>> a.__class__ = B
> >>> a.show()
> ALPHA
>
> Peter
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