[C++-sig] Pickle python subclass of C++ interface

John Reid j.reid at mail.cryst.bbk.ac.uk
Mon Jul 19 22:38:54 CEST 2010

John Reid wrote:
> John Reid wrote:
>> Ralf W. Grosse-Kunstleve wrote:
>>>> Ok I'll give that a whirl. I was hoping to avoid doing 
>>>> __getinitargs__()
>>>> for each subclass as I have quite a few of them. If I didn't have a C++
>>>> base class then the pickling would just work as is. There's no way I
>>>> can get back to that sort of situation with the C++ base class is 
>>>> there?
>>> I don't know, but there may be. One (totally untested) idea would be to
>>> give the base class __getstate__ and __setstate__ methods that 
>>> inspect the
>>> instance to re/store the state. Maybe you just need to return and 
>>> restore
>>> self.__dict__?
>>> I guess you could mix-in the __getstate__, __setstate__ methods.
>>> Ralf
>> I'm not sure what you mean by mix-in, but my first attempt involved 
>> defining pickle suite getstate() and setstate() methods. I did not 
>> define a getinitargs() method. Unfortunately when the derived object 
>> was unpickled, __init__ was called with no arguments. As far as I can 
>> see there's no way to use the boost.python pickle suite that does not 
>> involve a call to __init__() on the unpickled object.
>> I'll try having a go using the python pickling protocol's __reduce__() 
>> method.
> Ok this seems to work by injecting a __reduce__() method into the C++ 
> base class. Here ext is the extension module and A is the C++ base class:
> import ext, cPickle, logging, copy_reg
> def __newobj__(cls, *args):
>     return cls.__new__(cls, *args)
> def __reduce__(self):
>     return (
>         __newobj__,
>         (self.__class__,),
>         self.__dict__
>     )
> ext.A.__reduce__ = __reduce__
> class Derived(ext.A):
>     def __init__(self, init_arg):
>         self.data = init_arg
> derived = Derived(1)
> pickled_repr = cPickle.dumps(derived)
> unpickled = cPickle.loads(pickled_repr)
> assert unpickled.data == derived.data
OK it seems I was a bit optimistic. When I pass an unpickled object back 
to C++, I get this error:

Boost.Python.ArgumentError: Python argument types in
did not match C++ signature:

Does anyone have any ideas why my solution doesn't work when I try to 
pass an unpickled object back to C++? I guess I need to learn a bit more 
about how __new__() works.

> John.

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