Automated tests

At the moment there are only a few automated tests, so we need to start expanding it! It’s not easy to properly automatically test ML code, but there’s lots of opportunities for unit tests.

We use pytest. Here is a complete pytest API reference.

The tests have been configured to automatically run against the git checked out fastai repository and not pre-installed fastai.

Choosing which tests to run

To run all the tests:



   make test


   python test

To skip the integration tests in order to do quick testing while you work:

   pytest --skipint

If you need to skip a certain test module temporarily you can either tell pytest which tests to run explicitly, so for example to skip any test modules that contain the string link, you could run:

   pytest `ls -1 tests/*py | grep -v link`

To run an individual test file:

   pytest tests/

Run tests by keyword expressions:

   pytest -k "list and not listify" tests/

For example, if we have the following tests:

   def test_whatever():
   def test_listify(p, q, expected):
   def test_listy():

it will first select test_listify and test_listy, and then deselect test_listify, resulting in only the sub-test test_listy being run.

More ways:

For nuances of configuring pytest’s repo-wide behavior see collection.

Writing tests

When writing tests:

  • Avoid mocks; instead, think about how to create a test of the real functionality that runs quickly
  • Use module scope fixtures to run init code that can be shared amongst tests
  • Avoid pretrained models, since they have to be downloaded from the internet to run the test
  • Create some minimal data for your test, or use data already in repo’s data/ directory

Clearing state

CI builds and when isolation is important (against speed), cache should be cleared:

   pytest --cache-clear tests

Test order and repetition

It’s good to repeat the tests several times, in sequence, randomly, or in sets, to detect any potential inter-dependency and state-related bugs (tear down). And the straightforward multiple repetition is just good to detect some problems that get uncovered by randomness of DL.


  • Repeat tests:

     pip install pytest-repeat

    Now 2 new options becomes available:

    --count=COUNT         Number of times to repeat each test
    --repeat-scope={function,class,module,session} Scope for repeating tests


     pytest --count=10 tests/
     pytest --count=10 --repeat-scope=function tests
  • Run tests in a random order:

     pip install pytest-random-order

    Important: Presence of pytest-random-order will automatically randomize tests, no configuration change or command line options is required.

    XXX: need to find a package or write our own pytest extension to be able to randomize at will, since the two available modules that do that once installed force the randomization by default.

    As explained earlier this allows detection of coupled tests - where one test’s state affects the state of another. When pytest-random-order is installed it will print the random seed it used for that session, e.g:

     pytest tests
     Using --random-order-bucket=module
     Using --random-order-seed=573663

    So that if the given particular sequence fails, you can reproduce it by adding that exact seed, e.g.:

     pytest --random-order-seed=573663
     Using --random-order-bucket=module
     Using --random-order-seed=573663

    It will only reproduce the exact order if you use the exact same list of tests (or no list at all (==all)). Once you start to manually narrowing down the list you can no longer rely on the seed, but have to list them manually in the exact order they failed and tell pytest to not randomize them instead using --random-order-bucket=none, e.g.:

     pytest --random-order-bucket=none tests/ tests/ tests/

    To disable the shuffling for all tests:

     pytest --random-order-bucket=none

    By default --random-order-bucket=module is implied, which will shuffle the files on the module levels. It can also shuffle on class, package, global and none levels. For the complete details please see its documentation.

Randomization alternatives:

  • pytest-randomly

    This module has a very similar functionality/interface, but it doesn’t have the bucket modes available in pytest-random-order. It has the same problem of imposing itself once installed.

To GPU or not to GPU

On a GPU-enabled setup, to test in CPU-only mode add CUDA_VISIBLE_DEVICES="":

   CUDA_VISIBLE_DEVICES="" pytest tests/

To do the same inside the code of the test:

   fastai.torch_core.default_device = torch.device('cpu')

To switch back to cuda:

   fastai.torch_core.default_device = torch.device('cuda')

Make sure you don’t hard-code any specific device ids in the test, since different users may have a different GPU setup. So avoid code like:

   fastai.torch_core.default_device = torch.device('cuda:1')

which tells torch to use the 2nd GPU. Instead, if you’d like to run a test locally on a different GPU, use the CUDA_VISIBLE_DEVICES environment variable:

   CUDA_VISIBLE_DEVICES="1" pytest tests/

Report each sub-test name and its progress

For a single or a group of tests via pytest (after pip install pytest-pspec):

   pytest --pspec tests/
   pytest --pspec tests

For all tests via

   python test --addopts="--pspec"

This also means that meaningful names for each sub-test are important.

Output capture

During test execution any output sent to stdout and stderr is captured. If a test or a setup method fails, its according captured output will usually be shown along with the failure traceback.

To disable capturing and get the output normally use -s or --capture=no:

   pytest -s tests/

To send test results to JUnit format output:

   py.test tests --junitxml=result.xml

Color control

To have no color (e.g. yellow on white bg is not readable):

   pytest --color=no tests/

Sending test report to online pastebin service

Creating a URL for each test failure:

   pytest --pastebin=failed tests/

This will submit test run information to a remote Paste service and provide a URL for each failure. You may select tests as usual or add for example -x if you only want to send one particular failure.

Creating a URL for a whole test session log:

   pytest --pastebin=all tests/

Writing Tests

XXX: Needs to be written. Contributions are welcome.

Until then look at the existing tests.


When you run:

   make coverage

it will run the test suite directly via pytest and on completion open a browser to show you the coverage report, which will give you an indication of which parts of the code base haven’t been exercised by tests yet. So if you are not sure which new tests to write this output can be of great insight.

Remember, that coverage only indicated which parts of the code tests have exercised. It can’t tell anything about the quality of the tests. As such, you may have a 100% coverage and a very poorly performing code.


Skipping tests

This is useful when a bug is found and a new test is written, yet the bug is not fixed yet. In order to be able to commit it to the main repository we need make sure it’s skipped during make test.


  • A skip means that you expect your test to pass only if some conditions are met, otherwise pytest should skip running the test altogether. Common examples are skipping windows-only tests on non-windows platforms, or skipping tests that depend on an external resource which is not available at the moment (for example a database).

  • A xfail means that you expect a test to fail for some reason. A common example is a test for a feature not yet implemented, or a bug not yet fixed. When a test passes despite being expected to fail (marked with pytest.mark.xfail), it’s an xpass and will be reported in the test summary.

One of the important differences between the two is that skip doesn’t run the test, and xfail does. So if the code that’s buggy causes some bad state that will affect other tests, do not use xfail.


  • The whole test unconditionally:

     @pytest.mark.skip(reason="this bug needs to be fixed")
     def test_feature_x():

    or the xfail way:

     def test_feature_x():
  • Based on some internal check inside the test:

     def test_feature_x():
         if not has_something(): pytest.skip("unsupported configuration")

    or the whole module:

     import pytest
     if not pytest.config.getoption("--custom-flag"):
         pytest.skip("--custom-flag is missing, skipping tests", allow_module_level=True)

    or the xfail way:

     def test_feature_x():
         pytest.xfail("expected to fail until bug XYZ is fixed")
  • Skip all tests in a module if some import is missing:

     docutils = pytest.importorskip("docutils", minversion="0.3")
  • Skip if

     import sys
     @pytest.mark.skipif(sys.version_info < (3,6),
                        reason="requires python3.6 or higher")
     def test_feature_x():

    or the whole module:

     @pytest.mark.skipif(sys.platform == 'win32',
                        reason="does not run on windows")
     class TestPosixCalls(object):
         def test_feature_x(self):

More details, example and ways are here.

Getting reproducible results

In some situations you may want to remove randomness for your tests. To get identical reproducable results set, you’ll need to set num_workers=1 (or 0) in your DataLoader/DataBunch, and depending on whether you are using torch’s random functions, or python’s (numpy) or both:

  • torch RNG

     import torch
     torch.backends.cudnn.deterministic = True
  • python RNG


Notebook integration tests

The two places you should check for notebooks to test your code with are:

In each case, look for notebooks that have names starting with the application you’re working on - e.g. ‘text’ or ‘vision’.


The docs_src notebooks can be executed as a test suite: You need to have at least 8GB available on your GPU to run all of the tests. So make sure you shutdown any unnecessary jupyter kernels, so that the output of your nvidia-smi shows that you have at least 8GB free.

cd docs_src

To run a subset:

./ callback*

There are a lot more details on this subject matter in this document.


You can run each of these interactively in jupyter, or as CLI:

jupyter nbconvert --execute --ExecutePreprocessor.timeout=600 --to notebook examples/tabular.ipynb

This set is examples and there is no pass/fail other than visual observation.