The pytest Framework

The pytest framework is primarily used for running answer tests, where a simulation is run with two versions of enzo-e and their results are compared. This is useful for testing problems with no analytical solution or generally verifying that results from commonly run simulations don’t drift.

pytest is a Python-based framework for detecting and running a series of tests within a source code repository. When running pytest, the user can provide a directory in which pytest will look for files named test_*.py and run all functions within those files whose names start with “test”. pytest will run all tests and present a summary of which ones passed and failed. All functions that run without producing an error will be marked as passed.


pytest can be installed with pip or conda.

$ pip install pytest
$ conda install pytest

Answer Testing

Within enzo-e, we make use of the TestCase class to define a general EnzoETest class that will run a given simulation within a temporary directory and delete that directory once finished. This class and other useful answer testing functionality are located in the source in test/answer_tests/ All answer tests are located in the other files within the test/answer_tests directory.

Running the Answer Test Suite

The answer test suite is run in two stages. First, test answers must be generated from a version of the code known to function correctly. A git tag associated with the main repository marks a changeset for which the code is believed to produce good results. This tag is named gold-standard-#. To pull tags from the main repository and see which tags exist, do the following:

$ git fetch origin --tags
$ git tag

To generate test answers, use the highest numbered gold standard tag.

Configuring the Answer Test Suite

Before the answer tests can be run, a few environment variables must be set to configure behavior.

  • TEST_RESULTS_DIR: points to a directory in which answers will be stored

  • CHARM_PATH: points to the directory in which charmrun is located

  • ENZO_PATH: points to the enzo-e binary to use. If this is not specified, this defaults to the <PATH/TO/ENZO-E/REPO>/build/bin/enzo-e

  • GENERATE_TEST_RESULTS: “true” to generate test results, “false” to compare with existing results.

  • GRACKLE_INPUT_DATA_DIR: points to the directory where Grackle input files are installed. If not specified, then all tests involving Grackle will be skipped.

$ export TEST_RESULTS_DIR=~/enzoe_tests
$ export CHARM_PATH=~/local/charm-v7.0.0/bin

Generating Test Answers

First, check out the highest numbered gold standard tag and compile enzo-e.

$ git checkout gold-standard-1
$ ...compile enzo-e

Then, configure the test suite to generate answers by setting GENERATE_TEST_RESULTS to true.


Finally, run the test suite by calling pytest with the answer test directory.

$ pytest test/answer_tests
========================== test session starts ===========================
platform linux -- Python 3.9.13, pytest-7.1.2, pluggy-1.0.0
rootdir: /home/circleci/enzo-e
collected 1 item

test/answer_tests/ .                                   [100%]

=========================== 1 passed in 13.26s ===========================

Assuming there are no errors, this will run the simulations associated with the tests, perform the analysis required to produce the answers, save the answers to files, and report that all tests have passed.

Comparing Test Answers

Once test answers have been generated, the above steps need not be repeated until the gold standard tag has been updated. Now, any later version of the code can be run with the test suite to check for problems. Set the GENERATE_TEST_RESULTS environment variable to false to configure the test suite to compare with existing answers.

$ git checkout main
$ ...compile enzo-e
$ pytest test/answer_tests

Getting More Output from Pytest

By default, most output printed by enzo-e or the test scripts will be swallowed by pytest. When tests fail, the Python traceback may be shown, but not much else. There are various flags to increase the verbosity of pytest, but the -s flag will show all output, including from the simulation itself. The enzo-e answer test suite will also print out the values of all configuration variables when this flag is given.

$ pytest -s test/answer_tests

Creating New Answer Tests

This section follows the example of TestHLLCCloud in test/answer_tests/ Answer tests can be created by making a new Python file in the test/answer_tests directory with a name starting with ‘test_’ or by adding to an existing file if the test falls within the theme given by its name. If your test requires configuring a new simulation parameter file, see Create Input Parameters for the New Test. for information on setting that up.

The answer testing framework exists in test/answer_tests/ New test files created in the same directory can directly import from this file.

Creating a New Test Class

To make a new test, one must create a new Python class that subclasses the EnzoETest class. Three attributes must be defined within the class:

  • parameter_file: the relative path to the simulation parameter file from within the input directory.

  • max_runtime: the maximum runtime of the simulation in seconds. The simulation will be stopped and the test marked as failed if this is exceeded. Set this to something a bit longer than the typical runtime to detect when new changes have significantly altered the runtime. If not given, the max runtime is infinity.

  • ncpus: the number of processes with which to run the simulation.

from answer_testing import EnzoETest

class TestHLLCCloud(EnzoETest):
    parameter_file = "vlct/dual_energy_cloud/"
    max_runtime = 30
    ncpus = 1
Tests involving Grackle

If the class is associated with a test simulation that invokes Grackle, you need to annotate the class declaration with the uses_grackle decorator.

from answer_testing import EnzoETest, uses_grackle

class TestGrackleGeneral(EnzoETest):

For all classes annotated with this decorator:

  • the framework knows that it must make symbolic links to all files in the directory run by GRACKLE_INPUT_DATA_DIR before it runs the simulation associated with this class.

  • the testing framwork also knows to skip the associated test(s) if the GRACKLE_INPUT_DATA_DIR environment variable is unset.

If you forget to add this label, Enzo-E will not be able to locate the data file needed for Grackle (in a portable way). Thus, the associated simulation and test will fail.

Creating the Test Function

The code above configures the simulation associated with the test. The next step is to write a function which will be run after the simulation completes successfully. This is done by creating a class method within the test class. This function should only take the argument self (because it’s a class method) and nothing else. The function will be run from within the directory where the simulation was run, so it will be able to load any files that were output.

def test_hllc_cloud(self):
    fn = "hllc_cloud_0.0625/hllc_cloud_0.0625.block_list"
    assert os.path.exists(fn)

Tests are typically implemented with an assert or related statement. In the above example, we check for the existence of a file that should have been created by the simulation. This is not specifically an answer test as we are not comparing with results from another version of the code. However, these sorts of assertion checks can be included in your test function if they are useful for verifying proper running of the code.

Creating an Answer Test Function

To create an answer test that will automatically save data to files and compare with other files, we make use of the ytdataset_test Python decorator, also located in test/answer_tests/

from answer_testing import \
    EnzoETest, \
    ytdataset_test, \

We also import an assertion function that will check for relative closeness of values in an array.

The ytdataset_test decorator can then be put immediately above the definition of a test function. This wraps the test function in additional code that will save test files and run comparisons. With the ytdataset_test, one must also provide a function that will perform the comparison of results.

@ytdataset_test(assert_array_rel_equal, decimals=8)
def test_hllc_cloud(self):
    ds = yt.load("hllc_cloud_0.0625/hllc_cloud_0.0625.block_list")
    ad = ds.all_data()

    wfield = ("gas", "mass")
    data = {field[1]: ad.quantities.weighted_standard_deviation(field, wfield)
             for field in ds.field_list}

    return data

When using ytdataset_test decorator, a test function must return a dictionary of values. The values in the dictionary can be anything, e.g., numbers, string, arrays, etc. In the above example, we load a snapshot with yt and compute the weighted average and standard deviation (the weighted_standard_deviation function returns both) of all the fields on disk. We now only need to return that and the ytdataset_test wrapper will save a file named after the test function (in this case, ‘test_hllc_cloud.h5’ and will use the assert_array_rel_equal function to check that results agree to within 8 decimal places. Note, the NumPy testing module defines several other assertion functions which may be useful.

Including Additional Configuration Options

The easiest way to communicate additional configuration options is through environment variables. Once an environment variable is set (i.e., with export in bash), it can be seen by your test using the os.environ dict. Below, we use the USE_DOUBLE environment variable to determine whether enzo-e was compiled in single or double precision, and adjust the tolerance on the tests accordingly.

import os

use_double = os.environ.get("USE_DOUBLE", "false").lower() == "true"
if use_double:
    decimals = 12
    decimals = 6

# inside the TestHLLCCloud class
@ytdataset_test(assert_array_rel_equal, decimals=decimals)
def test_hllc_cloud(self):


Below are a few things to keep in mind when designing new tests.

Defining Multiple Test Functions within a Class

Multiple test functions can be implemented within the same answer test class. However, the test simulation will be run for each test. If you want to perform multiple checks on a long running simulation, it is a better idea to implement them all with separate asserts inside a single function.

Answer Test Functions Must Have Unique Names

Answer test functions that use the ytdataset_test wrapper must all have unique names. This is because each results file will be named with the name of the function itself.