Passing a MPI communicator to a SWIG-ed interface

This is not directly linked to MEDCoupling, but the following issue often arises when wrapping a C++ code to Python using the SWIG tool (for example, to be able to couple two codes in Python).

If you want to be able to pass a communicator defined on the Python side (typically with the Python module mpi4py) to your C++ code (which you have wrapped with SWIG), the following magic lines need to be inserted into your ".i" file for this to work correctly:

%include /usr/lib/python3/dist-packages/mpi4py/include/mpi4py/mpi4py.i
%mpi4py_typemap(Comm, MPI_Comm);
 
....
void method_taking_a_MPI_Comm(MPI_Comm mpicomm);
...

where the path to mpi4py.i needs to be adapted to your configuration.

In MEDCoupling DEC APIs (InterpKernelDEC and OverlapDEC constructors), the Python interface deals with that using the following trick to get the C++ address of the MPI communicator:

from mpi4py import MPI
MPI._addressof(mpicomm) # returns the C++ address of the MPI communicator