9. Fortran Unit Testing

9.1. Introduction

9.1.1. What is a unit test?

A unit test is a fast, self-verifying test of a small piece of code. A single unit test typically covers 10s to 100s of lines of code; a single function or small module, for example. It typically runs in milliseconds and produces a simple pass/fail result.

Unit tests:

  • Ensure that code remains correct as it is modified. In this respect, unit tests complement the CIME system tests.

  • Ensure that new code is correct.

  • Can help guide development, via test-driven development (TDD).

  • Provide executable documentation of the intended behavior of a piece of code.

  • Support development on your desktop machine.

9.1.2. Overview of unit test support in CIME

CIME comes with a set of tools to support building and running unit tests. These consist of:

  1. CMake tools to support building and running tests via CMake and CTest.

  2. A Python script that provides a simple front end for the CMake-based tests.

The Fortran unit tests use pFUnit, which is a Fortran testing framework that follows conventions of other xUnit frameworks. CIME’s support for pFUnit requires pFUnit version 4 or greater.

9.2. Running CIME’s Fortran unit tests

These instructions assume that you are using a machine that already has pFUnit installed, along with the necessary support in CIME. If that is not the case, see How to add unit testing support on your machine.

From the top-level CIME directory, you can run all of CIME’s Fortran unit tests by running:

> scripts/fortran_unit_testing/run_tests.py --build-dir MY_BUILD_DIR

You can replace MY_BUILD_DIR with a path to the directory where you would like the unit test build files to be placed. To ensure a completely clean build, use:

> scripts/fortran_unit_testing/run_tests.py --build-dir `mktemp -d ./unit_tests.XXXXXXXX`

Once you have built the unit tests (whether the build was successful or not), you can reuse the same build directory later to speed up the rebuild. There are a number of useful arguments to run_tests.py. For full usage information, run:

> scripts/fortran_unit_testing/run_tests.py --help

If your build is successful, you will get a message like this:

Running CTest tests for __command_line_test__/__command_line_test__.

This will be followed by a list of tests, with a Pass/Fail message for each, like these examples:

Test project /Users/sacks/cime/unit_tests.0XHUkfqL/__command_line_test__/__command_line_test__
      Start  1: avect_wrapper
 1/17 Test  #1: avect_wrapper ....................   Passed    0.02 sec
      Start  2: seq_map
 2/17 Test  #2: seq_map ..........................   Passed    0.01 sec
      Start  3: glc_elevclass
 3/17 Test  #3: glc_elevclass ....................   Passed    0.01 sec

You will also see a final message like this:

100% tests passed, 0 tests failed out of 17

These unit tests are run automatically as part of scripts_regression_tests on machines that have a serial build of pFUnit available for the default compiler.

9.3. How to add unit testing support on your machine

The following instructions assume that you have ported CIME to your machine by following the instructions in Porting and validating CIME on a new platform. If you have done that, you can add unit testing support by building pFUnit on your machine and then pointing to the build in your ** MACH*_*COMPILER.cmake** file. Those processes are described in the following sections.

9.3.1. Building pFUnit

Follow the instructions below to build pFUnit using the default compiler on your machine. That is the default for run_tests.py and that is required for scripts_regression_tests.py to run the unit tests on your machine. For the CMake step, we typically build with -DSKIP_MPI=YES, -DSKIP_OPENMP=YES and -DCMAKE_INSTALL_PREFIX set to the directory where you want pFUnit to be installed. (At this time, no unit tests require parallel support, so we build without MPI support to keep things simple.) Optionally, you can also provide pFUnit builds with other supported compilers on your machine.

  1. Obtain pFUnit from https://github.com/Goddard-Fortran-Ecosystem/pFUnit (see https://github.com/Goddard-Fortran-Ecosystem/pFUnit#obtaining-pfunit for details)

  2. Create a directory for the build and cd to that directory:

    > mkdir build-dir
    > cd build-dir
  3. Set up your environment to be similar to the environment used in CIME system builds. For example, load the appropriate compilers into your path. An easy way to achieve this is to run the following with an optional compiler argument:

    > $CIMEROOT/CIME/scripts/configure --mpilib mpi-serial

    Then source either ./.env_mach_specific.sh or ./.env_mach_specific.csh, depending on your shell.

    On some systems, you may need to explicitly set the FC and CC environment variables so that pFUnit’s CMake build picks up the correct compilers, e.g., with:

    > export FC=ifort
    > export CC=icc
  4. For convenience, set the PFUNIT environment variable to point to the location where you want to install pFUnit. For example (in bash):

    > export PFUNIT=$CESMDATAROOT/tools/pFUnit/pFUnit4.7.0_cheyenne_Intel19.1.1_noMPI_noOpenMP
  5. Configure and build pFUnit:

    > make -j 8
  6. Run pFUnit’s self-tests:

    > make tests
  7. Install pFUnit in the directory you specified earlier:

    > make install

You can repeat this process with different compiler environments. Make sure to choose a different installation directory for each build by setting the PFUNIT variable differently.

9.3.2. Adding to the appropriate cmake file

After you build pFUnit, tell CIME about your build or builds. To do this, specify the appropriate path using the PFUNIT_PATH CMake variable in the ** MACH*_*COMPILER.cmake** file. For a build with no MPI or openMP support (as recommended above), the block should look like this (with the actual path replaced with the PFUNIT path you specified when doing the build):

if (MPILIB STREQUAL mpi-serial AND NOT compile_threaded)
  set(PFUNIT_PATH "$ENV{CESMDATAROOT}/tools/pFUnit/pFUnit4.7.0_cheyenne_Intel19.1.1_noMPI_noOpenMP")

Once you have specified the path for your build(s), you should be able to run the unit tests by following the instructions in Running CIME’s Fortran unit tests.

9.4. How to write a new unit test


Need to write this section. This will draw on some of the information in sections 3 and 4 of https://github.com/NCAR/cesm_unit_test_tutorial (though without the clm and cam stuff).

It should also introduce the role of .pf files, which are referenced several paragraphs later as if already explained.

9.5. General guidelines for writing unit tests

Unit tests typically test a small piece of code, on the order of 10-100 lines, as in a single function or small class.

Good unit tests are “FIRST”: (https://pragprog.com/magazines/2012-01/unit-tests-are-first):

  • Fast (milliseconds or less). This means that, generally, they should not do any file i/o. Also, if you are testing a complex function, test it with a simple set of inputs rather than a 10,000-element array that will require a few seconds of runtime to process.

  • Independent. This means that test Y shouldn’t depend on some global variable that text X created. Such dependencies cause problems if the tests run in a different order, if one test is dropped, and so on.

  • Repeatable. This means, for example, that you shouldn’t generate random numbers in your tests.

  • Self-verifying. Don’t write a test that writes out its answers for manual comparison. Tests should generate an automatic pass/fail result.

  • Timely. Write the tests before the production code (TDD) or immediately afterwards - not six months later when it’s time to finally merge your changes onto the trunk and you have forgotten the details. Much of the benefit of unit tests comes from developing them concurrently with the production code.

Good unit tests test a single, well-defined condition. This generally means that you make a single call to the function or subroutine that you’re testing, with a single set of inputs. Usually you need to run multiple tests in order to test all of the unit’s possible behaviors.

Testing a single condition in each test makes pinpointing problems easier when a test fails. This also makes it easier to read and understand the tests, allowing them to serve as useful documentation of how the code should operate.

A good unit test has four distinct pieces:

  1. Setup: For example, creating variables that will be needed for the routine you’re testing. For simple tests, this piece may be empty.

  2. Exercise: Calling the routine you’re testing.

  3. Verify: Calling assertion methods (next section) to ensure that the results match what you expected.

  4. Teardown: For example, deallocating variables. For simple tests, this piece may be empty. If it is needed, however, it is best done in the special tearDown routine discussed in Defining a test class in order to define setUp and tearDown methods and More on test teardown.**

If you have many tests of the same subroutine, you may find quite a lot of duplication. It’s good practice to extract major areas of duplication to their own subroutines in the .pf file, which your tests can call. This aids the understandability and maintainability of your tests. pFUnit knows which subroutines are tests and which are “helper” routines because of the @Test directives: You only add a @Test directive for your tests, not for your helper routines.

9.6. More details on writing pFUnit-based unit tests

9.6.1. Assertion methods

pFUnit provides many assertion methods that you can use in the Verify step. Here are some of the most useful:

@assertEqual(expected, actual)

Ensures that expected == actual. Accepts an optional tolerance argument giving the tolerance for real-valued comparisons.

@assertLessThan(expected, actual)

Ensures that expected < actual.

@assertGreaterThan(expected, actual)

Ensures that expected > actual.

@assertLessThanOrEqual(expected, actual)

@assertGreaterThanOrEqual(expected, actual)


It is better to use the two-valued assertions above, if possible. They provide more information if a test fails.



Ensures that the result is not NaN or infinity.


This can be useful for failure checking - for example, when your function returns NaN to signal an error.

Comparison assertions accept an optional tolerance argument, which gives the tolerance for real-valued comparisons.

All of the assertion methods also accept an optional message argument, which prints a string if the assertion fails. If no message is provided, you will be pointed to the file and line number of the failed assertion.

9.6.2. Defining a test class in order to define setUp and tearDown methods

As noted in the comments in test_circle.pf, defining a test class is optional. However, defining a minimal test class as shown here with TestCircle allows you use some pFUnit features such as the setUp and tearDown methods.

type, extends(TestCase) :: TestCircle
   procedure :: setUp
   procedure :: tearDown
end type TestCircle

If you define this test class, you also need to:

  • Define setUp and tearDown subroutines. These can start out empty:

    subroutine setUp(this)
      class(TestCircle), intent(inout) :: this
    end subroutine setUp
    subroutine tearDown(this)
      class(TestCircle), intent(inout) :: this
    end subroutine tearDown
  • Add an argument to each subroutine of the class. By convention, this argument is named this.

Code in the setUp method is executed before each test. This is convenient if you need to do some setup that is the same for every test.

Code in the tearDown method is executed after each test. This is often used to deallocate memory. See More on test teardown for details.

You can add any data or procedures to the test class. Adding data is particularly useful, as this can be a way for the setUp and tearDown methods to interact with your tests: The setUp method can fill a class variable with data, which your tests can then use (accessed via this%somedata). Conversely, if you want the tearDown method to deallocate a variable, the variable cannot be local to your test subroutine. Instead, you make the variable a member of the class, so that the tearDown method can access it.

Here is an example. Say you have this variable in your test class:

real(r8), pointer :: somedata(:)

The setUp method can create somedata if it needs to be the same for every test.

Alternatively, it can be created in each test routine that needs it if it differs from test to test. (Some tests don’t need it at all.) In that situation, create it like this:

this%somedata(:) = [1,2,3,4,5]

Your tearDown method then can have code like this:

if (associated(this%somedata)) then
end if

9.6.3. More on test teardown

As stated in Defining a test class in order to define setUp and tearDown methods, code in the tearDown method is executed after each test, often to do cleanup operations.

Using the tearDown method is recommended because tests abort if an assertion fails. The tearDown method is still called, however, so teardown that needs to be done still gets done, regardless of pass/fail status. Teardown code might otherwise be skipped, which can lead other tests to fail or give unexpected results.

All of the tests in a single test executable run one after another. For CIME, this means all of the tests that are defined in all .pf files in a single test directory.

As a result, tests can interact with each other if you don’t clean up after yourself. In the best case, you might get a memory leak. In the worst case, the pass/fail status of tests depends on which other tests have run previously, making your unit tests unrepeatable and unreliable.

To avoid this:

  • Deallocate any pointers that your test allocates.

  • Reset any global variables to some known, initial state.

  • Do other, similar cleanup for resources that are shared by multiple tests.

In Fortran2003, allocatable variables are deallocated automatically when they go out of scope, but pointers are not. Explicitly deallocate any pointers that have been allocated, either in test setup or in the execution of the routine you are testing.

You might need to move some variables from subroutine-local to the class. This is because the tearDown method can access class instance variables, but not subroutine-local variables.

CIME makes extensive use of global variables that may be used directly or indirectly by a routine you are testing. If your test has allocated or modified any global variables, it is important to reset them to their initial state in the teardown portion of the test.

9.7. Finding more documentation and examples

9.7.1. More detailed examples in CIME

There are many examples of unit tests in CIME, some simple and some quite complex. You can find them by looking for files with the “.pf” extension:

> find . --name '*.pf'

You can also see examples of the unit test build scripts by viewing the CMakeLists.txt files throughout the source tree.

9.7.2. Other pFUnit documentation sources

Unfortunately, the documentation inside the pFUnit repository (in the documentation and Examples directories) is out-of-date (at least as of April, 2023): much of this documentation refers to version 3 of pFUnit, which differs in some ways from version 4. However, some working examples are provided in https://github.com/Goddard-Fortran-Ecosystem/pFUnit_demos.

9.7.3. Documentation of the unit test build system

The CMake build infrastructure is in $CIMEROOT/CIME/non_py/src/CMake.

The infrastructure for building and running tests with run_tests.py is in $CIMEROOT/scripts/fortran_unit_testing. That directory also contains general documentation about how to use the CIME unit test infrastructure (in the README file) and examples (in the Examples directory).