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