Adding Tests

If you are adding new features to manim, you should add appropriate tests for them. Tests prevent manim from breaking at each change by checking that no other feature has been broken and/or been unintentionally modified.

How Manim Tests

Manim uses pytest as its testing framework. To start the testing process, go to the root directory of the project and run pytest in your terminal. Any errors that occur during testing will be displayed in the terminal.

Some useful pytest flags:

  • -x will make pytest stop at the first failure it encounters

  • -s will make pytest display all the print messages (including those during scene generation, like DEBUG messages)

  • --skip_slow will skip the (arbitrarily) slow tests

  • --show_diff will show a visual comparison in case an unit test is failing.

How it Works

At the moment there are three types of tests:

  1. Unit Tests:

    Tests for most of the basic functionalities of manim. For example, there a test for Mobject, that checks if it can be added to a Scene, etc.

  2. Graphical unit tests:

    Because manim is a graphics library, we test frames. To do so, we create test scenes that render a specific feature. When pytest runs, it compares the last frame of every render to the control data; If it matches, the tests pass. If the test and control data differ, the tests fail. You can use --show_diff flag with pytest to visually see the differences.

  3. Videos format tests:

    As Manim is a video library, we have to test videos as well. Unfortunately, we cannot directly test video content as rendered videos can differ slightly depending on the system (for reasons related to ffmpeg). Therefore, we only compare video configuration values, exported in .json.


The manim/tests directory looks like this:

├── control_data
│   ├── graphical_units_data
│   │   ├── creation
│   │   │   ├── DrawBorderThenFillTest.npy
│   │   │   ├── FadeInFromDownTest.npy
│   │   │   ├── FadeInFromLargeTest.npy
│   │   │   ├── FadeInFromTest.npy
│   │   │   ├── FadeInTest.npy
│   │   │   ├── ...
│   │   ├── geometry
│   │   │   ├── AnnularSectorTest.npy
│   │   │   ├── AnnulusTest.npy
│   │   │   ├── ArcBetweenPointsTest.npy
│   │   │   ├── ArcTest.npy
│   │   │   ├── CircleTest.npy
│   │   │   ├── CoordinatesTest.npy
│   │   │   ├── ...
│   │   ├── graph
│   │   │   ├── ...
|   |   |   | ...
│   └── videos_data
│       ├── SquareToCircleWithDefaultValues.json
│       └── SquareToCircleWithlFlag.json
├── helpers
│   ├──
│   ├──
│   └──
├── test_graphical_units
│   ├──
│   ├──
│   ├──
│   ├──
│   ├──
│   ├──
│   ├──
│   ├──
│   ├──
│   └──
├── test_logging
│   ├──
│   ├── expected.txt
│   ├── testloggingconfig.cfg
│   └──
├── test_scene_rendering
│   ├──
│   ├──
│   ├──
│   ├── standard_config.cfg
│   └──
└── utils

The Main Directories

  • control_data/:

    The directory containing control data. control_data/graphical_units_data/ contains the expected and correct frame data for graphical tests, and control_data/videos_data/ contains the .json files used to check videos.

  • test_graphical_units/:

    Contains graphical tests.

  • test_scene_rendering/:

    For tests that need to render a scene in some way, such as tests for CLI flags (end-to-end tests).

  • utils/:

    Useful internal functions used by pytest.


    fixtures are not contained here, they are in

  • helpers/:

    Helper functions for developers to setup graphical/video tests.

Adding a New Test

Unit Tests

Pytest determines which functions are tests by searching for files whose names begin with “test_”, and then within those files for functions beginning with “test” and classes beginning with “Test”. These kinds of tests must be in tests/ (e.g. tests/

Graphical Unit Test

The test must be written in the correct file (i.e. the file that corresponds to the appropriate category the feature belongs to) and follow the structure of unit tests.

For example, to test the Circle VMobject which resides in manim/mobject/, add the CircleTest to test/

In, add:

class CircleTest(Scene):
    def construct(self):
        circle = Circle()

Scene names follow the syntax: <thing_to_test>Test. In the example above, we are testing whether Circle properly shows up with the generic Animation and not any specific animation.


If the file already exists, edit it and add the test within the file. The Scene will be tested thanks to the GraphicalUnitTester that lives in tests/utils/ Import it with from ..utils.GraphicalUnitTester import GraphicalUnitTester.

To test all the scenes in the module, we do the following:

@pytest.mark.parametrize("scene_to_test", get_scenes_to_test(__name__), indirect=False)
def test_scene(scene_to_test, tmpdir, show_diff):
    GraphicalUnitTester(scene_to_test[1], MODULE_NAME, tmpdir).test(show_diff=show_diff)

The first line is a pytest decorator. It is used to run a test function several times with different parameters. Here, we pass in all the scenes as arguments.


If you run pytest now, you will get a FileNotFound error. This is because you have not created control data for your test.

Next, we’ll want to create control data for CircleTest. In tests/, there exists the function, set_test_scene, for this purpose.

It will look something like this:

class CircleTest(Scene):
    def construct(self):
        circle = Circle()

set_test_scene(CircleTest, "geometry")

set_test_scene takes two parameters: the scene to test, and the module name. You can generate the test data by running the file (it suffices to type the name of the file in the terminal; you do not have to run it like how you would normally run manim files). It will automatically generate the control data in the right directory (in this case, tests/control_data/graphical_units_data/geometry/CircleTest.npz).

Please make sure to add the control data to git as soon as it is produced with git add <your-control-data.npz>. However, do not include changes to the template script ( in your pull request so that others may continue to use the unmodified file to generate their own tests.

Videos tests

To test videos generated, we use the decorator tests.utils.videos_tester.video_comparison:

    "SquareToCircleWithlFlag.json", "videos/simple_scenes/480p15/SquareToCircle.mp4"
def test_basic_scene_l_flag(tmp_path, manim_cfg_file, simple_scenes_path):
    scene_name = "SquareToCircle"
    command = [
    out, err, exit_code = capture(command)
    assert exit_code == 0, err


assert exit*\ code == 0, err is used in case of the command fails to run. The decorator takes two arguments: json name and the path to where the video should be generated, starting from the media/ dir.

Note the fixtures here:

  • tmp_path is a pytest fixture to get a tmp_path. Manim will output here, according to the flag --media_dir.

  • manim_cfg_file fixture that return a path pointing to test_scene_rendering/standard_config.cfg. It’s just to shorten the code, in the case multiple tests need to use this cfg file.

  • simple_scenes_path same as above, except for test_scene_rendering/

You have to generate a .json file first to be able to test your video. To do that, use helpers.save_control_data_from_video.

For instance, a test that will check if the l flag works properly will first require rendering a video using the -l flag from a scene. Then we will test (in this case, SquareToCircle), that lives in test_scene_rendering/ Change directories to tests/, create a file (e.g. create\ that you will remove as soon as you’re done. Then run:

save_control_data_from_video("<path-to-video>", "SquareToCircleWithlFlag.json")

Running this will save control_data/videos_data/SquareToCircleWithlFlag.json, which will look like this:

    "name": "SquareToCircleWithlFlag",
    "config": {
        "codec_name": "h264",
        "width": 854,
        "height": 480,
        "avg_frame_rate": "15/1",
        "duration": "1.000000",
        "nb_frames": "15"

If you have any questions, please don’t hesitate to ask on Discord, in your pull request, or in an issue.