[C++-sig] Re: creating an instance of a class_<> object in c+ + and exporting it to python

[Max Khesin]

I believe that you will not see the "dummy" method by doing dir(). IIRC
boost.python hijacks __getattr__ to delegate the lookup to the c++
implemented code. Just try a.dummy(), does it work?
max

> -----Original Message-----
> From: Francois Ostiguy [mailto:ostiguy at fnal.gov]
> Sent: Monday, August 16, 2004 5:51 PM
> To: c++-sig at python.org
> Subject: Re: [C++-sig] Re: creating an instance of a class_<> 
> object in
> c++ and exporting it to python
> 
> 
> 
> >
> > As Mike and I wrote, just do this:
> >
> >   object PythonInstanceOfA  = py_get_existing();
> >
> 
> Unfortunately, that does not seem to do quite what I want.
> I constructed a simple complete toy example to demonstrate.
> 
> Note the following:
> 
> (1) creating python_object_a forces an undesired instantiation of
>     a type A (i.e. in addition to the the existing instance a)
> 
> (2) if the interface defines a python object without
>      an __init__ function, the result is a runtime error:
>      "Object cannot by created from python."
> 
> (3) if you compile the example, here is what you get when the
>     module is imported from python
> 
> 
>   Python 2.3.3 (#1, Jul  7 2004, 13:43:38)
>   [GCC 3.3.3] on linux2
>   Type "help", "copyright", "credits" or "license" for more 
> information.
>   >>> from inject import *
>   >>> a
>   <inject.A object at 0xb733b10c>
>   >>> dir()
>   ['A', '__builtins__', '__doc__', '__name__', 'a']
>   >>> dir (a)
>   ['A', '__class__', '__delattr__', '__dict__', '__doc__',
>   '__getattribute__', '__hash__', '__init__', '__instance_size__',
>   '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__',
>   '__setattr__', '__str__', '__weakref__']
>   >>>
> 
>   Note that the object instance "a" has been injected. However, for a
>   reason that escapes me, the method "dummy" which is
>   part of the specified interface is not correctly exported.
> 
>   Again, my goal is to wrap an existing instance of A, without
>   having to create a spurious object (basically without having to
>   call the A constructor more than once).
> 
>   Comments, suggestions ???
> 
> -Francois
> 
> --------------------------------------------------------------
> --------------
> Dr. Jean-Francois OSTIGUY                              voice: 
> (630) 840-2231
> Beam Physics Dept MS220                                  FAX: 
> (630) 840-6039
> Fermi National Accelerator Laboratory                email: 
> ostiguy at fnal.gov
> Batavia IL 60510-0500                           
> WWW:www-ap.fnal.gov/~ostiguy
> 
> 
> /////// *************** example: inject.cpp **************** /////////
> 
> #include <boost/python.hpp>
> 
> class A {
> public:
>   A(int d) { _dummy = d; }
>   int dummy() { return _dummy;}
> private:
> 
>   int _dummy;
> };
> 
> A* a = 0;
> 
> A* get_existing() { return a; }
> 
> void inject_object();
> 
> using namespace boost::python;
> 
> class_ <A, A*,  boost::noncopyable>* PythonTypeAPtr;
> 
> BOOST_PYTHON_MODULE( inject )
> {
> 
> 
>     static class_ <A, A*,  boost::noncopyable> 
> PythonTypeA("A", init<int>());
> 
>    //*** note: using no_init here will cause a runtime error !!
> 
>    PythonTypeA.def("A", &A::dummy);
> 
>    /***** this does not seem to export the dummy function to 
> python. Why ?
> 
>    PythonTypeAPtr = &PythonTypeA;
> 
>     inject_object();
> 
> }
> 
> void  inject_object() {
> 
>  a = new A(99);
> 
>  object py_get_existing =
>      make_function( &get_existing, return_value_policy<
> reference_existing_object>() );
> 
>  object python_object_a = (*PythonTypeAPtr)(2); // <<<<<<==========
>  //****** instantiation of a spurious object
> 
>  python_object_a = py_get_existing();
> 
> 
>  handle<> main_module ( boost::python::borrowed(
> PyImport_AddModule("__main__") )  );
>  handle<> main_dict   ( boost::python::borrowed(
> PyModule_GetDict(main_module.get()) ));
>  PyDict_SetItemString  ( main_dict.get(), "a",  
> python_object_a.ptr() );
> 
> }
> 
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