Make assertions before the "act" step

[fconil]

Hi,

I want to test a function that takes to 2 input videos.

Those videos must be similar : same width, height and fps properties. I wrote an assertion function that makes those checks.

Is it wrong to make those assertions before the act step ?

If this is wrong, what is the right way to proceed ?

def assert_similar_videos(video_path_1, video_path_2):
    # Get video properties ...
    assert width1 == width2 and height1 == height2
    assert fps1 == fps2

def test_video_treatment():
    # Test the input videos
    assert_similar_videos(video_1, video_2)

    # Do the treatment
    video_treatment(video_1, video_2, parameters, output_video)

    # Test the output video
    assert output_video.exists()
    assert_similar_videos(video_1, output_video)

Thanks for your help
Françoise

[The-Compiler]

I think that's perfectly fine! I sometimes explicitly mark those as # sanity check so it's clearer that this is something that should never fail, and is just to ensure the test itself is actually working properly.

You could also possibly do this in a fixture, as in:

@pytest.fixture
def treatment_videos():
    # get video1 and video2 from somewhere
    assert_similar_videos(video_1, video_2)
    return video_1, video_2

def test_video_treatment(treatment_videos):
    video1, video2  = treatment_videos
    ...

which would then result in the test being marked as E (error) instead of F (failed) if that fails.

[nicoddemus]

I sometimes explicitly mark those as # sanity check so it's clearer that this is something that should never fail

I second that. This is very useful and I personally consider it a best practice.

It is much easier to reason when a test sudden starts to fail if the cause is a broken sanity check, than having to understand/debug the whole test to realize some precondition has changed (some fixture has changed the values it returns, for example).

[fconil]

I am cleaning an existing code and the arguments are not exactly the same but I understand that I would be better to write the check as a fixture.
Thanks a lot for your prompt reply