yt includes a testing suite which one can run on the codebase to assure that no breaks in functionality have occurred. This testing suite is based on the Nose testing framework. The suite consists of two components, unit tests and answer tests. Unit tests confirm that an isolated piece of functionality runs without failure for inputs with known correct outputs. Answer tests verify the integration and compatibility of the individual code unit by generating output from user-visible yt functions and comparing and matching the results against outputs of the same function produced using older versions of the yt codebase. This ensures consistency in results as development proceeds.

The testing suite should be run locally by developers to make sure they aren’t checking in any code that breaks existing functionality. To further this goal, an automatic buildbot runs the test suite after each code commit to confirm that yt hasn’t broken recently. To supplement this effort, we also maintain a continuous integration server that runs the tests with each commit to the yt version control repository.

Unit Testing

What do Unit Tests Do

Unit tests are tests that operate on some small set of machinery, and verify that the machinery works. yt uses the Nose framework for running unit tests. In practice, what this means is that we write scripts that assert statements, and Nose identifies those scripts, runs them, and verifies that the assertions are true and the code runs without crashing.

How to Run the Unit Tests

One can run the unit tests very straightforwardly from any python interpreter that can import the yt module:

import yt

If you are developing new functionality, it is sometimes more convenient to use the Nose command line interface, nosetests. You can run the unit tests using nose by navigating to the base directory of the yt mercurial repository and invoking nosetests:

$ cd $YT_HG
$ nosetests

where $YT_HG is the path to the root of the yt mercurial repository.

If you want to specify a specific unit test to run (and not run the entire suite), you can do so by specifying the path of the test relative to the $YT_HG/yt directory – note that you strip off one yt more than you normally would! For example, if you want to run the plot_window tests, you’d run:

$ nosetests yt/visualization/tests/test_plotwindow.py

How to Write Unit Tests

yt provides several pieces of testing assistance, all in the yt.testing module. Describing them in detail is somewhat outside the scope of this document, as in some cases they belong to other packages. However, a few come in handy:

  • fake_random_ds() provides the ability to create a random dataset, with several fields and divided into several different grids, that can be operated on.
  • assert_equal() can operate on arrays.
  • assert_almost_equal() can operate on arrays and accepts a relative allowable difference.
  • assert_allclose_units() raises an error if two arrays are not equal up to a desired absolute or relative tolerance. This wraps numpy’s assert_allclose to correctly verify unit consistency as well.
  • amrspace() provides the ability to create AMR grid structures.
  • expand_keywords() provides the ability to iterate over many values for keywords.

To create new unit tests:

  1. Create a new tests/ directory next to the file containing the functionality you want to test and add an empty __init__.py file to it.
  2. Inside that directory, create a new python file prefixed with test_ and including the name of the functionality.
  3. Inside that file, create one or more routines prefixed with test_ that accept no arguments. The test function should do some work that tests some functionality and should also verify that the results are correct using assert statements or functions.
  4. Tests can yield a tuple of the form function, argument_one, argument_two, etc. For example yield my_test, 'banana', 2.0 would be captured by nose and the my_test function will be run with the provided arguments.
  5. Use fake_random_ds to test on datasets, and be sure to test for several combinations of nproc, so that domain decomposition can be tested as well.
  6. Test multiple combinations of options by using the expand_keywords() function, which will enable much easier iteration over options.

For an example of how to write unit tests, look at the file yt/data_objects/tests/test_covering_grid.py, which covers a great deal of functionality.

Debugging failing tests

When writing new tests, often one exposes bugs or writes a test incorrectly, causing an exception to be raised or a failed test. To help debug issues like this, nose can drop into a debugger whenever a test fails or raises an exception. This can be accomplished by passing --pdb and --pdb-failures to the nosetests executable. These options will drop into the pdb debugger whenever an error is raised or a failure happens, respectively. Inside the debugger you can interactively print out variables and go up and down the call stack to determine the context for your failure or error.

nosetests --pdb --pdb-failures

In addition, one can debug more crudely using print statements. To do this, you can add print statements to the code as normal. However, the test runner will capture all print output by default. To ensure that output gets printed to your terminal while the tests are running, pass -s to the nosetests executable.

Lastly, to quickly debug a specific failing test, it is best to only run that one test during your testing session. This can be accomplished by explicitly passing the name of the test function or class to nosetests, as in the following example:

$ nosetests yt.visualization.tests.test_plotwindow:TestSetWidth

This nosetests invocation will only run the tests defined by the TestSetWidth class.

Finally, to determine which test is failing while the tests are running, it helps to run the tests in “verbose” mode. This can be done by passing the -v option to the nosetests executable.

All of the above nosetests options can be combined. So, for example to run the TestSetWidth tests with verbose output, letting the output of print statements come out on the terminal prompt, and enabling pdb debugging on errors or test failures, one would do:

$ nosetests --pdb --pdb-failures -v -s yt.visualization.tests.test_plotwindow:TestSetWidth

Answer Testing

What do Answer Tests Do

Answer tests test actual data, and many operations on that data, to make sure that answers don’t drift over time. This is how we test frontends, as opposed to operations, in yt.

How to Run the Answer Tests

The very first step is to make a directory and copy over the data against which you want to test. Currently, we test:


  • D9p_500/10MpcBox_HartGal_csf_a0.500.d


  • sizmbhloz-clref04SNth-rs9_a0.9011/sizmbhloz-clref04SNth-rs9_a0.9011.art


  • ShockCloud/id0/Cloud.0050.vtk
  • MHDBlast/id0/Blast.0100.vtk
  • RamPressureStripping/id0/rps.0062.vtk
  • MHDSloshing/virgo_low_res.0054.vtk


  • RadAdvect/plt00000
  • RadTube/plt00500
  • StarParticles/plrd01000


  • TurbBoxLowRes/data.0005.3d.hdf5
  • GaussianCloud/data.0077.3d.hdf5
  • IsothermalSphere/data.0000.3d.hdf5
  • ZeldovichPancake/plt32.2d.hdf5
  • KelvinHelmholtz/data.0004.hdf5


  • DD0010/moving7_0010 (available in tests/ in the yt distribution)
  • IsolatedGalaxy/galaxy0030/galaxy0030
  • enzo_tiny_cosmology/DD0046/DD0046
  • enzo_cosmology_pluts/DD0046/DD0046


  • radio_fits/grs-50-cube.fits
  • UnigridData/velocity_field_20.fits


  • WindTunnel/windtunnel_4lev_hdf5_plt_cnt_0030
  • GasSloshingLowRes/sloshing_low_res_hdf5_plt_cnt_0300


  • IsothermalCollapse/snap_505
  • IsothermalCollapse/snap_505.hdf5
  • GadgetDiskGalaxy/snapshot_200.hdf5


  • InteractingJets/jet_000002
  • WaveDarkMatter/psiDM_000020

Halo Catalog

  • owls_fof_halos/groups_001/group_001.0.hdf5
  • owls_fof_halos/groups_008/group_008.0.hdf5
  • gadget_fof_halos/groups_005/fof_subhalo_tab_005.0.hdf5
  • gadget_fof_halos/groups_042/fof_subhalo_tab_042.0.hdf5
  • rockstar_halos/halos_0.0.bin


  • c5/c5.h5m


  • output_00080/info_00080.txt


  • halo1e11_run1.00400/halo1e11_run1.00400
  • agora_1e11.00400/agora_1e11.00400
  • TipsyGalaxy/galaxy.00300


  • snapshot_033/snap_033.0.hdf5

These datasets are available at http://yt-project.org/data/.

Next, modify the file ~/.yt/config to include a section [yt] with the parameter test_data_dir. Set this to point to the directory with the test data you want to test with. Here is an example config file:

test_data_dir = /Users/tomservo/src/yt-data

More data will be added over time. To run the answer tests, you must first generate a set of test answers locally on a “known good” revision, then update to the revision you want to test, and run the tests again using the locally stored answers.

Let’s focus on running the answer tests for a single frontend. It’s possible to run the answer tests for all the frontends, but due to the large number of test datasets we currently use this is not normally done except on the yt project’s contiguous integration server.

$ cd $YT_HG
$ nosetests --with-answer-testing --local --local-dir $HOME/Documents/test --answer-store --answer-name=local-tipsy yt.frontends.tipsy

This command will create a set of local answers from the tipsy frontend tests and store them in $HOME/Documents/test (this can but does not have to be the same directory as the test_data_dir configuration variable defined in your .yt/config file) in a file named local-tipsy. To run the tipsy frontend’s answer tests using a different yt changeset, update to that changeset, recompile if necessary, and run the tests using the following command:

$ nosetests --with-answer-testing --local --local-dir $HOME/Documents/test --answer-name=local-tipsy yt.frontends.tipsy

The results from a nose testing session are pretty straightforward to understand, the results for each test are printed directly to STDOUT. If a test passes, nose prints a period, F if a test fails, and E if the test encounters an exception or errors out for some reason. Explicit descriptions for each test are also printed if you pass -v to the nosetests executable. If you want to also run tests for the ‘big’ datasets, then you will need to pass --answer-big-data to nosetests. For example, to run the tests for the OWLS frontend, do the following:

$ nosetests --with-answer-testing --local --local-dir $HOME/Documents/test --answer-store --answer-big-data yt.frontends.owls

How to Write Answer Tests

Tests can be added in the file yt/utilities/answer_testing/framework.py . You can find examples there of how to write a test. Here is a trivial example:

class MaximumValueTest(AnswerTestingTest):
    _type_name = "MaximumValue"
    _attrs = ("field",)
    def __init__(self, ds_fn, field):
        super(MaximumValueTest, self).__init__(ds_fn)
        self.field = field

    def run(self):
        v, c = self.ds.find_max(self.field)
        result = np.empty(4, dtype="float64")
        result[0] = v
        result[1:] = c
        return result

    def compare(self, new_result, old_result):
        assert_equal(new_result, old_result)

What this does is calculate the location and value of the maximum of a field. It then puts that into the variable result, returns that from run and then in compare makes sure that all are exactly equal.

To write a new test:

  • Subclass AnswerTestingTest
  • Add the attributes _type_name (a string) and _attrs (a tuple of strings, one for each attribute that defines the test – see how this is done for projections, for instance)
  • Implement the two routines run and compare The first should return a result and the second should compare a result to an old result. Neither should yield, but instead actually return. If you need additional arguments to the test, implement an __init__ routine.
  • Keep in mind that everything returned from run will be stored. So if you are going to return a huge amount of data, please ensure that the test only gets run for small data. If you want a fast way to measure something as being similar or different, either an md5 hash (see the grid values test) or a sum and std of an array act as good proxies. If you must store a large amount of data for some reason, try serializing the data to a string (e.g. using numpy.ndarray.dumps), and then compressing the data stream using zlib.compress.
  • Typically for derived values, we compare to 10 or 12 decimal places. For exact values, we compare exactly.

How To Write Answer Tests for a Frontend

To add a new frontend answer test, first write a new set of tests for the data. The Enzo example in yt/frontends/enzo/tests/test_outputs.py is considered canonical. Do these things:

  • Create a new directory, tests inside the frontend’s directory.
  • Create a new file, test_outputs.py in the frontend’s tests directory.
  • Create a new routine that operates similarly to the routines you can see in Enzo’s output tests.
    • This routine should test a number of different fields and data objects.
    • The test routine itself should be decorated with @requires_ds(test_dataset_name). This decorator can accept the argument big_data=True if the test is expensive. The test_dataset_name should be a string containing the path you would pass to the yt.load function. It does not need to be the full path to the dataset, since the path will be automatically prepended with the location of the test data directory. See The Configuration File for more information about the test_data-dir configuration option.
    • There are small_patch_amr and big_patch_amr routines that you can yield from to execute a bunch of standard tests. In addition we have created sph_answer which is more suited for particle SPH datasets. This is where you should start, and then yield additional tests that stress the outputs in whatever ways are necessary to ensure functionality.

If you are adding to a frontend that has a few tests already, skip the first two steps.

How to Write Image Comparison Tests

We have a number of tests designed to compare images as part of yt. We make use of some functionality from matplotlib to automatically compare images and detect differences, if any. Image comparison tests are used in the plotting and volume rendering machinery.

The easiest way to use the image comparison tests is to make use of the GenericImageTest class. This class takes three arguments:

  • A dataset instance (e.g. something you load with yt.load or data_dir_load)
  • A function the test machinery can call which will save an image to disk. The test class will then find any images that get created and compare them with the stored “correct” answer.
  • An integer specifying the number of decimal places to use when comparing images. A smaller number of decimal places will produce a less stringent test. Matplotlib uses an L2 norm on the full image to do the comparison tests, so this is not a pixel-by-pixel measure, and surprisingly large variations will still pass the test if the strictness of the comparison is not high enough.

You must decorate your test function with requires_ds, otherwise the answer testing machinery will not be properly set up.

Here is an example test function:

from yt.utilities.answer_testing.framework import \
    GenericImageTest, requires_ds, data_dir_load

from matplotlib import pyplot as plt

def test_my_ds():
    ds = data_dir_load(my_ds)

    def create_image(filename_prefix):
        plt.plot([1, 2], [1, 2])
    test = GenericImageTest(ds, create_image, 12)

    # this ensures the test has a unique key in the
    # answer test storage file
    test.prefix = "my_unique_name"

    # this ensures a nice test name in nose's output
    test_my_ds.__name__ = test.description

    yield test

Another good example of an image comparison test is the PlotWindowAttributeTest defined in the answer testing framework and used in yt/visualization/tests/test_plotwindow.py. This test shows how a new answer test subclass can be used to programmatically test a variety of different methods of a complicated class using the same test class. This sort of image comparison test is more useful if you are finding yourself writing a ton of boilerplate code to get your image comparison test working. The GenericImageTest is more useful if you only need to do a one-off image comparison test.

Enabling Answer Tests on Jenkins

Before any code is added to or modified in the yt codebase, each incoming changeset is run against all available unit and answer tests on our continuous integration server. While unit tests are autodiscovered by nose itself, answer tests require definition of which set of tests constitute to a given answer. Configuration for the integration server is stored in tests/tests.yaml in the main yt repository:

      - yt/frontends/artio/tests/test_outputs.py
# ...
      - '-v'
      - '-s'

Each element under answer_tests defines answer name (local_artio_000 in above snippet) and specifies a list of files/classes/methods that will be validated (yt/frontends/artio/tests/test_outputs.py in above snippet). On the testing server it is translated to:

$ nosetests --with-answer-testing --local --local-dir ... --answer-big-data \
   --answer-name=local_artio_000 \

If the answer doesn’t exist on the server yet, nosetests is run twice and during first pass --answer-store is added to the commandline.

Updating Answers

In order to regenerate answers for a particular set of tests it is sufficient to change the answer name in tests/tests.yaml e.g.:

--- a/tests/tests.yaml
+++ b/tests/tests.yaml
@@ -25,7 +25,7 @@
     - yt/analysis_modules/halo_finding/tests/test_rockstar.py
     - yt/frontends/owls_subfind/tests/test_outputs.py

-  local_owls_000:
+  local_owls_001:
     - yt/frontends/owls/tests/test_outputs.py


would regenerate answers for OWLS frontend.

When adding tests to an existing set of answers (like local_owls_000 or local_varia_000), it is considered best practice to first submit a pull request adding the tests WITHOUT incrementing the version number. Then, allow the tests to run (resulting in “no old answer” errors for the missing answers). If no other failures are present, you can then increment the version number to regenerate the answers. This way, we can avoid accidently covering up test breakages.

Adding New Answer Tests

In order to add a new set of answer tests, it is sufficient to extend the answer_tests list in tests/tests.yaml e.g.:

--- a/tests/tests.yaml
+++ b/tests/tests.yaml
@@ -60,6 +60,10 @@
     - yt/analysis_modules/absorption_spectrum/tests/test_absorption_spectrum.py:test_absorption_spectrum_non_cosmo
     - yt/analysis_modules/absorption_spectrum/tests/test_absorption_spectrum.py:test_absorption_spectrum_cosmo

+  local_gdf_000:
+    - yt/frontends/gdf/tests/test_outputs.py

Restricting Python Versions for Answer Tests

If for some reason a test can be run only for a specific version of python it is possible to indicate this by adding a [py2] or [py3] tag. For example:

      - yt/test_A.py  # [py2]
      - yt/test_B.py  # [py3]

would result in test_A.py being run only for python2 and test_B.py being run only for python3.