[gdal-dev] More consistent use of pytest's parameterization in autotests

[Daniel Evans]

> Additionally, these loops fix the order that the checks are made, and bugs can hide in test order dependency.

As we're on the topic of pytest, I'll mention fixtures as a handy
feature for test setup: https://docs.pytest.org/en/6.2.x/fixture.html

I discovered a while back that some parts of the GDAL test suite break
if test order changes, because Test B relies implicitly some state
being left over from Test A. This prevents some possible improvements
like parallelising the test suite for speed (at least, without careful
manual organisation of what can be done in parallel). It was also a
little annoying trying to debug tests that started segfaulting when
the order changed!

Regards,
Daniel

On Mon, 20 Feb 2023 at 15:43, Sean Gillies <sean.gillies at gmail.com> wrote:
>
> Hi all,
>
> I just saw a maintainer recommend to a contributor that the contributor loop over test cases from within a test function and it prompted me to speak up about a better practice: using pytest parameterization https://docs.pytest.org/en/6.2.x/parametrize.html#pytest-mark-parametrize-parametrizing-test-functions.
>
> Using a loop, like at https://github.com/OSGeo/gdal/blob/master/autotest/gcore/gdal_stats.py#L660, can hide bugs and add friction to development. In that particular case, there are 5 different fill values to be checked. If there are different code paths for each of these values (a worst case scenario) and each has a small bug, you will have to run the whole test suite 5 times to expose the 5 bugs one at a time. Additionally, these loops fix the order that the checks are made, and bugs can hide in test order dependency. Ideally, GDAL's tests run in random order.
>
> In a nutshell, you hoist the loop and list out into a test function decorator and then pytest discovers and runs all the separate cases. Pytest doesn't stop on the first failure in the list unless you explicitly tell it to do so.
>
> # Original GDAL test code
> def test_fill_value():
>     for fill_val in [0, 1, 32767, 32768, 65535]:
>         func(fill_val)
>         assert something
>
> # With pytest parameterization
> @pytest.mark.parametrize("fill_val", [0, 1, 32767, 32768, 65535])
> def test_fill_value(fill_val):
>         func(fill_val)
>         assert something
>
> I'm not proposing rewrites of old tests, but I think it is worth adopting a better practice for tests now and in the future.
>
> --
> Sean Gillies
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