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

[Austin Bingham]

If it's any help, I wrote a small bit on the details of writing
converters. It might help clarify some of the mechanics which, I
agree, are a bit mysterious:

  http://misspent.wordpress.com/2009/09/27/how-to-write-boost-python-converters/

Austin

On Wed, Sep 30, 2009 at 11:40 PM, James Amundson <amundson at fnal.gov> wrote:
> 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);
> }
>
>
>
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