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

[Ralf W. Grosse-Kunstleve]

lvalue_from_pytype only works for passing by value or const references/pointers, but that seems to match your case,
since you MPI_Comm is passed by value to your sayhello().

However, wrapping extern "C" isn't portable, AFAIK. You'll need a thin C++ wrapper for each of the C functions.

Did you consider writing a thin object-oriented wrapper around the raw MPI interfaces? Then it should be very straightforward to bind the wrapper with Boost.Python, and you'll have a more intuitive Python interface.
E.g. make a class that owns MPI_Comm, passed as boost::python::object, and then give it a .sayhello() method using the comm data member.




----- Original Message ----
From: James Amundson <amundson at fnal.gov>
To: cplusplus-sig at python.org
Sent: Wednesday, September 30, 2009 2:40:29 PM
Subject: [C++-sig] boost python to/from python type conversion

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);
}



_______________________________________________
Cplusplus-sig mailing list
Cplusplus-sig at python.org
http://mail.python.org/mailman/listinfo/cplusplus-sig