# Usage

The important thing to remember is that pytest-mpi assists with running tests when pytest is run under MPI, rather than launching pytest under MPI. To actually run the tests under MPI, you will want to run something like:

    $ mpirun -n 2 python -m pytest --with-mpi

Note that by default the MPI tests are not run—this makes it easy to run the non-MPI parts of a test suite without having to worry about installing MPI and mpi4py.

An simple test using the mpi marker managed by pytest-mpi is:

    import pytest
    @pytest.mark.mpi
    def test_size():
        from mpi4py import MPI
        comm = MPI.COMM_WORLD
        assert comm.size > 0

This test will be automatically be skipped unless –with-mpi is used. We can also specify a minimum number of processes required to run the test:

    import pytest
    @pytest.mark.mpi(min_size=2)
    def test_size():
        from mpi4py import MPI
        comm = MPI.COMM_WORLD
        assert comm.size >= 2

There are also mpi_skip, for when a test should not be run under MPI (e.g. it causes a lockup or segmentation fault), and mpi_xfail, for when a test should succeed when run normally, but fail when run under MPI.