[Python-Dev] How to pickle class derived from c++ extension

[Martin Drautzburg]

I understand that I can picke an extension class written in C/C++ by providing 
a __reduce__() method along with __getstate__()/__setstate__(). While I still 
havent gotten this to work, my main question is:

How could I possibly pickle an object of a  python class which is derived from 
the C++ extension?

It seems that I can define 

	>>> class Bar(list):
	...      pass

and add more attributes

	>>> l=Bar()
	>>> l.x=11

and __reduce__() will show the "x" attribute

	>>> l.__reduce__()
	(<function _reconstructor at 0xb7e2cf0c>, (<class '__main__.Bar'>, 
<type 'list'>, []), {'x': 11})

But this does not seem to work with my extension class Foo. I defined a 
__getstate__() method and __reduce__() indeed shows me some state.  But if I 
create a derived class Bar on the Python side and an object bar as an 
instance of that class, and add an "x" attribute to that bar object, then 
__reduce__ing that object shows nothing about the "x" attribute.

This is in a way undestandable, as __reduce__() eventually just calls 
__getstate__() and the only implementation it can find is in my Foo extension 
class, which knows nothing abpout the Bar derived class let alone its "x" 
attribute.

I would like to have __reduce__() do it the pyhon way as far as it cat get, 
and then call some magic method of my C++ class to pickle the "C++ part" of 
an object. Is there a way to achieve this? The "list" class seems to have 
something that my Foo class does not have. What is this?

Or of course if there is a better way, to picke objects of classes which are 
derived from C++ extensions I'd be happy to hear about it.