All code in HOOMD must be tested to ensure that it operates correctly.

Unit tests check that basic functionality works, one class at a time. Unit tests assume internal knowledge about how classes work and may use unpublished APIs to stress test all possible input and outputs of a given class in order to exercise all code paths. For example, test that the box class properly wraps vectors back into the minimum image. Unit tests should complete in a fraction of a second.

System integration tests check that many classes work together to produce correct output. These tests are black box tests and should only use user-facing APIs to provide inputs and check for correct outputs. For example, test that the hard sphere HPMC simulation executes for several steps. System integration tests may take several seconds.

Long running tests check for correct behavior, but require up to a minute to execute. Mark long running tests with the validate label.

Validation tests rigorously check that simulations sample the correct statistical ensembles. These tests take hours to execute on many CPU cores or GPUs. Find HOOMD’s validation tests in the hoomd-validation repository.


The following Python packages are required to execute tests. Some tests will be skipped when optional requirements are missing.

Running tests#

Change to the build directory and execute the following commands to run the tests:

  • ctest - Executes C++ tests

  • python3 -m pytest hoomd

pytest may be run outside the build directory by:

  • Passing a full path to the build: python3 -m pytest <build-directory>/hoomd

  • After installing to an environment: python3 -m pytest --pyargs hoomd


python3 -m pytest --pyargs hoomd tests the hoomd installation it finds by import hoomd, which may not be the one you just built. You must also change to a directory outside the source, otherwise import hoomd attempts to import the uncompiled source.

See also

See the pytest documentation for information on how to control output, select specific tests, and more.

Running tests with MPI#

When ENABLE_MPI=ON, CTest will execute some tests with mpirun -n 1, some with -n 2 and some with -n 8. Make sure your test environment (e.g. interactive cluster job) is correctly configured before running ctest.

pytest tests may also be executed with MPI with 2 ranks. pytest does not natively support MPI. Execute it with the provided wrapper script in the build directory:

mpirun -n 2 build/hoomd/hoomd/pytest/pytest-openmpi.sh -v -x build/hoomd

The wrapper script displays the outout of rank 0 and redirects rank 1’s output to a file. Inspect this file when a test fails on rank 1. This will result in an MPI_ABORT on rank 0 (assuming the -x argument is passed):

cat pytest.out.1


Pass the -x option to prevent deadlocks when tests fail on only 1 rank.


The provided wrapper script supports OpenMPI.

Executing long runing tests#

Longer running tests do not execute by default. Run these with the --validate command line option to pytest:

$ python3 -m pytest build/hoomd --validate -m validate
$ mpirun -n 2 hoomd/pytest/pytest-openmpi.sh build/hoomd -v -x -ra --validate -m validate


The -m validate option selects only the long running tests.


To run long running tests on an installed hoomd package, you need to specify additional options:

python3 -m pytest --pyargs hoomd -p hoomd.pytest_plugin_validate -m validate --validate

Implementing tests#

Most tests should be implemented in pytest. HOOMD’s test rig provides a device fixture that most tests should use to cache the execution device across multiple tests and reduce test execution time.


Add any new test_*.py files to the list in the corresponding CMakeLists.txt file.

Only add C++ tests for classes that have no Python interface or otherwise require low level testing. If you are unsure, please check with the lead developers prior to adding new C++ tests. Add new validation tests to the hoomd-validation repository.