Re: [Boost-users] Copy of a C++ object extended with Boost.Python

Subject: Re: [Boost-users] Copy of a C++ object extended with Boost.Python
From: 4ian (dark4ian_at_[hidden])
Date: 2009-08-25 05:52:54

Thanks for you response !
When you say "that one wrapper helper", are you speaking about the
object class of Boost.python, or about a home made class ?

On 24 août, 22:01, OvermindDL1 <overmind..._at_[hidden]> wrote:
> On Mon, Aug 24, 2009 at 1:06 PM, 4ian<dark4..._at_[hidden]> wrote:
> > I have a simple C++ object, exposed to python with Boost.python, like
> > this :
>
> > -------------------------------------------------
> > BOOST_PYTHON_MODULE(MyModule)
> > {
> >   class_<MyObject>("MyObject")...
> > }
> > -------------------------------------------------
>
> > I call then python, making an instance of MyObject accessible :
>
> > -------------------------------------------------
> > MyObject anInstance;
>
> > object main_module((handle<>( borrowed( PyImport_AddModule
> > ( "__main__" ) ) ) ) );
> > object main_namespace = main_module.attr( "__dict__" );
> > object myModule( (handle<>(PyImport_ImportModule("MyModule"))) );
> > main_namespace["anInstance"] = ptr(&anInstance);
>
> > try
> > {
> >   handle<> ignored(( PyRun_String( "anInstance.aNewMember = 1;print
> > anInstance.aNewMember;",
> >                                    Py_file_input,
> >                                    main_namespace.ptr(),
> >                                    main_namespace.ptr() ) ) );
> > }
> > catch ( error_already_set )
> > {
> >   PyErr_Print();
> > }
> > -------------------------------------------------
>
> > You can observe that I've added a member ( aNewMember ) to MyObject
> > instance.
> > But if I make a copy of my object :
>
> > -------------------------------------------------
> > MyObject anotherInstance = anInstance;
>
> > main_namespace["anotherInstance"] = ptr(&anotherInstance);
>
> > try
> > {
> >   handle<> ignored(( PyRun_String( "print
> > anotherInstance.aNewMember;",
> >                                    Py_file_input,
> >                                    main_namespace.ptr(),
> >                                    main_namespace.ptr() ) ) );
> > }
> > catch ( error_already_set )
> > {
> >   PyErr_Print();
> > }
> > -------------------------------------------------
>
> > I have a error saying that MyObject hasn't any "aNewMember" member.
> > What I have added with python has not been copied/transfered.
> > Is there a workaround to add permanently some new members to wrapped
> > objects ?
>
> You are adding an entry to the __dict__ of the wrapper class that
> contains the MyObject, but on the line MyObject anotherInstance =
> anInstance; you are creating a copy of only the C++ object, not the
> wrapper object that has the __dict__.  You need to make a copy of it
> through the Python bindings, not directly on the C++ copy-constructor
> because then you are just copying the MyObject, not the wrapper
> object.
>
> I have worked around it by putting a dict in MyObject and exposing it
> to python as __dict__ though, just make sure you overload your copy
> constructor so you copy the dict too and not just make a copy of the
> dict handle, that is important.  I did this work-around in an odd way,
> not sure it would work directly in Boost.Python, but I would think so.
>  I think the proper way in Boost.Python would be to (multi-)subclass
> your MyObject from that one wrapper helper so you can get the python
> self from your class's this, then you can copy the self (not the
> handle, again this is important) to the new one as well.
> _______________________________________________
> Boost-users mailing list
> Boost-us..._at_[hidden]http://lists.boost.org/mailman/listinfo.cgi/boost-users