Using maybempi in a library

A library that supports both serial and MPI runs imports the MPI module from maybempi wherever it would import mpi4py’s:

from maybempi import MPI


def default_comm():
    return MPI.COMM_WORLD

MPI is decided once per process, so the library, the libraries it uses and the application always agree, even if each of them imports it.

Type annotations

For type checkers, annotate with mpi4py’s types and fall back to the stand-in at run time:

from typing import TYPE_CHECKING

import maybempi

if TYPE_CHECKING:
    from mpi4py import MPI
else:
    MPI = maybempi.get_mpi()

Comm = "MPI.Comm | maybempi.SerialComm"

MPI.Intracomm | None also works at run time with the stand-in, so annotations evaluated at definition time do not break.

Serial-only behavior

Use maybempi.is_serial rather than comm.Get_size() == 1 when the code must know that MPI is not running at all, for example before calling into a C library that needs MPI_Init:

if maybempi.is_serial(comm):
    run_serial_solver()
else:
    run_distributed_solver(comm)

Tests

Tests can force either mode without a launcher: maybempi.get_mpi(False) returns the stand-in, and MAYBEMPI=0 keeps a whole mpirun job serial. To run a test suite on several ranks:

mpiexec -n 2 python -m pytest tests/mpi