__slots__ and multiple inheritance

[Dave Reed]

On Saturday 14 December 2002 15:53, Carl Banks wrote:
> Dave Reed wrote:
> > 
> > __slots__ doesn't seem to work as I expect when I use 
mixins/multiple
> > inheritance:
> > 
> > I've got a BaseSQL class (see code at bottom of message) in which I
> > define __slots__ (I started using it so pychecker didn't complain
> > about me using the instance variables from the other base class). I
> > left out "values" from __slots__, but it still lets me assign to
> > self.values.
> > 
> > Also, If I put the same __slots__ in the StudentSQL class, I get the
> > error:
> > 
> >    class Student(BaseSQL, StudentSQL):
> > TypeError: multiple bases have instance lay-out conflict
> > 
> > Can anyone explain to me how this should work?
> 
> It doesn't work at all.  It seems kind of stupid at first, but you
> can't inherit from two classes with slots (well, unless those classes'
> slots are all inherited from a common base class).


I wasn't very clear - without the __slots__ in the the StudentSQL
class, then it happily lets me access self.values in the BaseSQL class
even though 'values' is not in the BaseSQL __slots__ list (and the
other instance variables are). I expected to get an error when I tried
to set self.values, but didn't.

Regarding the second point:

I read the following from Guido's "Unifying Types and Classes in
Python 2.2" but I don't quite understand it.

  There's no check to prevent name conflicts between the slots defined
  in a class and the slots defined in its base classes. If a class
  defines a slot that's also defined in a base class, the instance
  variable defined by the base class slot is inaccessible (except by
  retrieving its descriptor directly from the base class; this could
  be used to rename it). Doing this renders the meaning of your
  program undefined; a check to prevent this may be added in the
  future.
 
It appears to me that you could have __slots__ in both base classes
as long as they intersection of the lists is empty (I'm pretty certain
I tried that and it didn't work either).

> The reason for this is that slots have fixed positions in the object's
> low-level structure.  One base class puts its slots at position x in
> the structure; another base class also puts slots at position x.  You
> can't multiply inherit from both of them because of the clash.

Thanks,
Dave