Down casting Python objects

Wed Mar 10 02:04:04 EST 2010

En 2010年3月09日 18:26:52 -0300, Andrey Fedorov <anfedorov at gmail.com> 
escribió:
> So I have `x', a instance of class `Foo'. I also have class `Bar', a 
> class
> extending `Foo' with a couple of methods. I'd like to "down cast" x as
> efficiently as possible. Is it OK to just set `x.__class__ = Bar' and 
> expect
> things to work OK in all major versions of CPython?

If the assignment x.__class__ = Bar succeeds, yes, that's enough. But you 
may not be able to reassign __class__ in some cases (e.g. Foo defines 
__slots__ but Bar doesn't, so you have a layout conflict).
Of course, you always have to make sure Bar invariants hold on this 
particular instance...
I must ask, why you didn't create a Bar instance in the first place?
-- 
Gabriel Genellina