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Though I haven't checked out the latest from CVS, I'm at a 
loss to explain the results Lorien is getting. In 
particular, Boost.Python class instances have a __class__ 
attribute, and Boost.Python classes have a __bases__ 
attribute that is a tuple. Isn't that all that should be 
needed for isinstance() to work properly?...

Aha, now I remember: The first base class in the list 
controls which metaclass object gets control when a derived 
class is created. So the declaration of MyDerivedClass 
below actually creates a regular Python class with a base 
that is a Boost.Python class. At that point, everything is 
beyond my control. To get it to work, at least begin by 
swapping the order of the base classes.

I wonder if it would make sense to always defer to the 
first extension type's type (if any) for subclass creation? 
It's not a general solution (i.e. what if you have multiple 
extension types in the __bases__ tuple?) but at least it 
would handle this case. Surely it's less likely that a 
regular Python class can make any sane use of methods from 
an extension class.

-Dave