Kevin Jacobs <jacobs@penguin.theopalgroup.com> writes:
> On 30 Oct 2002, Michael Hudson wrote:
> > For moderately nefarious reasons[1] I've being trying to write a
> > metaclass whose instances have writable __bases__. This in itself
> > isn't so hard, but having assigments to __bases__ "do the right thing"
> > has eluded me, basically because I can't seem to affect the mro.
>
> The mro is an internal data structure of new-style classes, so redefining
> mro() doesn't change the values used.
Yeah, I noticed that eventually.
> Here is my (non-working version) that
> attempts to re-assign the class of an object, although it fails on a layout
> violation with Python 2.2.2.
>
> def base_getter(cls):
> return cls.__my_bases__
>
> def base_setter(cls,bases):
> if not bases:
> bases = (object,)
> metaclass = getattr(cls, '__metaclass__', type)
> new_cls = metaclass(cls.__name__, bases, dict(cls.__dict__))
> cls.__class__ = new_cls
>
> class MetaBase(type):
> __bases__ = property(base_getter,base_setter)
>
> def __new__(cls, name, bases, ns):
> ns['__my_bases__'] = tuple(bases)
> return super(MetaBase, cls).__new__(cls, name, bases, ns)
>
> class Foo(object):
> __metaclass__ = MetaBase
> class Baz(object): pass
>
> Foo.__bases__ = Foo.__bases__ + (Baz,)
I don't think this has a hope of working does it? This approach would
need to rebind "Foo" in the last line, no?
> Which results in:
> TypeError: __class__ assignment: 'Foo' object layout differs from 'MetaBase'
>
> I haven't looked into why this is being flagged as a layout error, though
> my first instinct is to say that the check is too conservative in this case.
> I'll think about it more and dig into the code.
I think the error message gives it away: in "cls.__class__ = new_cls",
cls.__class__ is MetaBase, new_cls is the new Foo.
I guess one way of doing this would be to reimplement mro resolution
and so on in Python, which would be annoying and inefficient. Hmm.
Cheers,
M.
--
[3] Modem speeds being what they are, large .avi files were
generally downloaded to the shell server instead[4].
[4] Where they were usually found by the technical staff, and
burned to CD. -- Carlfish, asr
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