[Python-Dev] metaclass insanity

[Michael Hudson]

Guido van Rossum <guido@python.org> writes:

> > > The best thing to do would perhaps to make __mro__ assignable, but
> > > with a check that ensures the above constraint.  I think I'd take a
> > > patch for that.
> > 
> > Shouldn't be too hard.
> > 
> > > I'd also take a patch for assignable __bases__.  Note that there are
> > > constraints between __bases__ and __base__.
> > 
> > Should assigning to __bases__ automatically tweak __mro__ and
> > __base__?  Guess so.
> 
> Yes.  Note that changing __base__ should not be done lightly --
> basically, the old and new base must be layout compatible, exactly
> like for assignment to __class__.

OK.  I can crib code from type_set_class, I guess.  Or one could just
allow assignment to __bases__ when __base__ doesn't change?  __base__
is object for the majority of new-style classes, isn't it?

Brr.  There's a lot I don't know about post 2.2 typeobject.c.

> > What would assigning to __base__ do in isolation?
> > Perhaps that shouldn't be writeable.
> 
> Perhaps it could be writable when __bases__ is a 1-tuple.

Don't see the point of that.

> But it's fine if it's not writable.

Easier :)

> > > I'd also take a patch for assignable __name__.
> > 
> > This is practically a one-liner, isn't it?  Not hard, anyway.
> 
> Probably.  Can't remember why I didn't do it earlier.

It's a bit more complicated than that.

What's the deal wrt. dots in tp_name?  Is there any way for a user
defined class to end up called "something.somthing_else"?

Oh, and while we're at it, here's a bogosity:

>>> class C(object):
...  pass
... 
>>> C.__module__ 
'__main__'
>>> C.__module__ = 1
>>> C.__module__ 
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
AttributeError: __module__

caused by lax testing in type_set_module.

> > And there was me wondering what I was going to do this evening.
> 
> I don't have that problem -- a Zope customer problem was waiting for
> me today. :-(

Well, I didn't get it finished either.  Fiddly, this stuff.  Maybe by
tomorrow.

Cheers,
M.

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