TypeError: multiple bases have instance lay-out conflict

[Jeremy Hylton]

"robin and jim" <robinjim at earthlink.net> wrote in message news:<4GQ79.11079$LO1.881271 at newsread2.prod.itd.earthlink.net>...
> The following 2 class definitions:
> 
> class Spam(object):
>    __slots__ = ['notes']
> 
> class Eggs(Spam, dict):
>    pass
> 
> produce the following error concerning class Eggs:
> 
> TypeError: multiple bases have instance lay-out conflict
> 
> Could someone explain this error message and why the above Eggs class
> definition is prohibited.

The builtin type dict and a new-style class with slots both define
custom object layouts at the C level.  The C layout of Spam has a
specific slot for the notes attribute, while the dict type has its
hashtable.  There's not way to resolve this conflict, because the
layout of the C structs is fixed for each type.

Jeremy