When do I get "multiple bases have instance lay-out conflict" error?

[Harish K Vishwanath]

Hello all,

When do we actually see the error :

TypeError: Error when calling the metaclass bases
    multiple bases have instance lay-out conflict


I searched the web and I could not find a correct guideline as to when such
an error could arise. One of the places said that "A new style class cannot
inherit from more than one python built-in class". That is.,

>>> class A(dict,list):
...     pass
wouldn't work.

But, I have an instance where I am getting the above error even when I dont
inherit from multiple built-in types.

>>> sys.__version__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute '__version__'
>>> sys.version
'2.5 (r25:51908, Sep 19 2006, 09:52:17) [MSC v.1310 32 bit (Intel)]'

Using Py 2.5.2

>>> import persistent
>>> persistent.Persistent.__bases__
(<type 'object'>,)

Persistent is ZOPE/ZODB implementation. It inherits no built-in types

>>> Exception.__bases__
(<type 'exceptions.BaseException'>,)
>>> BaseException.__bases__
(<type 'object'>,)

Exception also inherits no built-in types.

>>> class A(Exception,persistent.Persistent):
...     pass
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Error when calling the metaclass bases
    multiple bases have instance lay-out conflict

Still declaration of class A fails. Any idea? What is the exact rule when
python throws up "    multiple bases have instance lay-out conflict" error?

Thanks in advance for the help.
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