[Python-Dev] metaclass insanity

[Guido van Rossum]

> > > 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?

But if you derive from a builtin type (e.g. list or dict), __base__
will be that.

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

Me too. :-)

> > > 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 :)

Agreed.

> > > > 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"?

I hope not.  The dots are for extensions living inside a package;
everything before the last dot ends up as __module__.

> 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.

Oops.  Can you fix it?  Or are there complications?  Seems to be
broken in 2.2 too.

> > > 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.

Great!

I'll be offline Friday through Monday -- going to a weekend conference.

--Guido van Rossum (home page: http://www.python.org/~guido/)