5. Test Suite

The OpenMC test suite consists of two parts, a regression test suite and a unit test suite. The regression test suite is based on regression or integrated testing where different types of input files are configured and the full OpenMC code is executed. Results from simulations are compared with expected results. The unit tests are primarily intended to test individual functions/classes in the OpenMC Python API.

5.1. Prerequisites

  • The test suite relies on the third-party pytest package. To run either or both the regression and unit test suites, it is assumed that you have OpenMC fully installed, i.e., the openmc executable is available on your PATH and the openmc Python module is importable. In development where it would be onerous to continually install OpenMC every time a small change is made, it is recommended to install OpenMC in development/editable mode. With setuptools, this is accomplished by running:

    python -m pip install -e .[test]
  • The test suite requires a specific set of cross section data in order for tests to pass. A download URL for the data that OpenMC expects can be found within tools/ci/download-xs.sh. Once the tarball is downloaded and unpacked, set the OPENMC_CROSS_SECTIONS environment variable to the path of the cross_sections.xml file within the unpacked data.

  • In addition to the HDF5 data, some tests rely on ENDF files. A download URL for those can also be found in tools/ci/download-xs.sh. Once the tarball is downloaded and unpacked, set the OPENMC_ENDF_DATA environment variable to the top-level directory of the unpacked tarball.

  • Some tests require NJOY to preprocess cross section data. The test suite assumes that you have an njoy executable available on your PATH.

5.2. Running Tests

To execute the Python test suite, go to the tests/ directory and run:


If you want to collect information about source line coverage in the Python API, you must have the pytest-cov plugin installed and run:

pytest --cov=../openmc --cov-report=html

To execute the C++ test suite, go to your build directory and run:


If you want to view testing output on failure run:

ctest --output-on-failure

5.3. Possible Reasons for Test Failures

You may find that when you run the test suite, not everything passes. First, make sure you have satisfied all the prerequisites above. After you have done that, consider the following:

  • When building OpenMC, make sure you run CMake with -DCMAKE_BUILD_TYPE=Debug. Building with a release build will result in some test failures due to differences in which compiler optimizations are used.

  • Because tallies involve the sum of many floating point numbers, the non-associativity of floating point numbers can result in different answers especially when the number of threads is high (different order of operations). Thus, if you are running on a CPU with many cores, you may need to limit the number of OpenMP threads used. It is recommended to set the OMP_NUM_THREADS environment variable to 2.

  • Recent versions of NumPy use instruction dispatch that may generate different results depending the particular ISA that you are running on. To avoid issues, you may need to disable AVX512 instructions. This can be done by setting the NPY_DISABLE_CPU_FEATURES environment variable to “AVX512F AVX512_SKX”. When NumPy/SciPy are built against OpenBLAS, you may also need to limit the number of threads that OpenBLAS uses internally; this can be done by setting the OPENBLAS_NUM_THREADS environment variable to 1.

5.4. Generating XML Inputs

Many of the regression tests rely on the Python API to build an appropriate model. However, it can sometimes be desirable to work directly with the XML input files rather than having to run a script in order to run the problem/test. To build the input files for a test without actually running the test, you can run:

pytest --build-inputs <name-of-test>

5.5. Adding C++ Unit Tests

The C++ test suite uses Catch2 integrated with CTest. Each header file should have a corresponding test file in tests/cpp_unit_tests/. If the test file does not exist run:

touch test_<name-of-header-file>.cpp

The file must be added to the CMake build system in tests/cpp_unit_tests/CMakeLists.txt. test_<name-of-header-file> should be added to TEST_NAMES.

To add a test case to test_<name-of-header-file>.cpp ensure catch2/catch_test_macros.hpp is included. A unit test can then be added using the TEST_CASE macro and the REQUIRE assertion from Catch2.

5.6. Adding Tests to the Regression Suite

To add a new test to the regression test suite, create a sub-directory in the tests/regression_tests/ directory. To configure a test you need to add the following files to your new test directory:

  • OpenMC input XML files, if they are not generated through the Python API

  • test.py - Python test driver script; please refer to other tests to see how to construct. Any output files that are generated during testing must be removed at the end of this script.

  • inputs_true.dat - ASCII file that contains Python API-generated XML files concatenated together. When the test is run, inputs that are generated are compared to this file.

  • results_true.dat - ASCII file that contains the expected results from the test. The file results_test.dat is compared to this file during the execution of the python test driver script. When the above files have been created, generate a results_test.dat file and copy it to this name and commit. It should be noted that this file should be generated with basic compiler options during openmc configuration and build (e.g., no MPI, no debug/optimization).

For tests using the Python API, both the inputs_true.dat and results_true.dat files can be generated automatically in the correct format via:

pytest --update <name-of-test>

In addition to this description, please see the various types of tests that are already included in the test suite to see how to create them. If all is implemented correctly, the new test will automatically be discovered by pytest.