[C++-sig] boost python to/from python type conversion

[James Amundson]

Hi,

I'm trying to write to/from type converters for my code, and I'm stuck. 
My case doesn't map well to any of those in the boost python 
documentation. I'm trying to pass objects of type MPI_Comm (really a C 
type!) back and forth with mpi4py (which uses cython internally.) The 
mpi4py author has provided a working example using manual wrapping. I am 
attaching it to the end of this message. On the C++ side I can extract a 
point to the MPI_Comm object from a Python object like so:

PyObject* py_obj;
// obviously, the value of py_obj has to be set somewhere...
MPI_Comm *comm_p = PyMPIComm_Get(py_obj);

I thought it would be simple to write a type converter to do that, but I 
don't understand the mechanics. This example doesn't seem to map onto 
the lvalue_from_pytype paradigm, or the scitbx container_conversion 
paradigm.

Any suggestions?

Thanks,
Jim Amundson

-----------------------------------
// working example with manual conversion

#include <mpi.h>
#include <iostream>

static void sayhello(MPI_Comm comm)
{
   if (comm == MPI_COMM_NULL) {
     std::cout << "You passed MPI_COMM_NULL !!!" << std::endl;
     return;
   }
   int size;
   MPI_Comm_size(comm, &size);
   int rank;
   MPI_Comm_rank(comm, &rank);
   int plen; char pname[MPI_MAX_PROCESSOR_NAME];
   MPI_Get_processor_name(pname, &plen);
   std::cout <<
     "Hello, World! " <<
     "I am process " << rank <<
     " of " << size <<
     " on  " << pname <<
     "." << std::endl;
}


#include <boost/python.hpp>
#include <mpi4py/mpi4py.h>
using namespace boost::python;

static void hw_sayhello(object py_comm)
{
   PyObject* py_obj = py_comm.ptr();
   MPI_Comm *comm_p = PyMPIComm_Get(py_obj);
   if (comm_p == NULL) throw_error_already_set();
   sayhello(*comm_p);
}

BOOST_PYTHON_MODULE(helloworld)
{
   if (import_mpi4py() < 0) return; /* Python 2.X */

   def("sayhello", hw_sayhello);
}