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

[Even Rouault]

Hi Sean,

I fully agree that pytest.mark.parametrize() is cleaner and the way to 
go, and I use it extensively in new tests. For what you were referring 
too, this was a change in an existing test that used the old for looping 
habits, so I felt it was a bit too much to ask the contributor to 
refactor it.

I've tried to enhance the existing documentation page related to tests 
with recommendations from this email thread and other things from 
related RFCs: https://github.com/OSGeo/gdal/pull/7277

Even

Le 20/02/2023 à 16:42, Sean Gillies a écrit :
> 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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-- 
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