Importing the C API fails on Windows

I am trying to build a C++ Python extension (with pybind11) that uses MPI and mpi4py on Windows. I am working in a conda environment and I installed mpi4py as follows:

conda install mpi4py -c conda-forge --yes

The following:

// import the mpi4py API
if (import_mpi4py() < 0) {
  throw std::runtime_error("Could not load mpi4py API.");
}

throws the exception with traceback:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Users\IEUser\VeloxChemMP\build_dbg_14.27\lib\python3.8\site-packages\veloxchem\__init__.py", line 2, in <module>
    from .veloxchemlib import AtomBasis
ImportError: Could not load mpi4py API.

Running mpiexec -n 4 python -c “from mpi4py import MPI; comm = MPI.COMM_WORLD; print(comm.Get_rank())” works as expected. I am not sure whether this is a bug or some silly mistake I am making.

This issue was migrated from moved from BitBucket #177

[dalcinl]

I’ll merge the bugfix/msmpi branch soon.

[robertodr]

@robertodr Thanks! About a new mpi4py release, it will happen eventually, but not immediately. However, we could easily patch the current conda-forge package to include this fix. Would that be convenient to you?

Remember that if you pass -DPyMPI_HAVE_MPI_Message to your compiler flags, the current release mpi4py release still works.

Or you can keep using the following in your code:

#include <mpi.h>

#ifdef MSMPI_VER
#define PyMPI_HAVE_MPI_Message 1
#endif

#include <mpi4py/mpi4py.h>

I will keep using the #ifdef ... #define ... in my code, keeping an eye out for new releases. Thanks!