Python testing with parameterization from function return

Question

I've been trying to solve a python test using pytest but have not been able to find an example configuration that works - though some are close. Here is my case study:

@pytest.fixture
def vil_check():
   code
   return [(v1,v2,v3), (...), (...)]

@pytest.mark.parameterize("v1,v2,v3", vil_check):
def test_one(v1,v2,v3):
      assert v1 < 2
      assert v2 > 5
      ....

I'm trying to follow this example:

@pytest.mark.parametrize("test_input,expected", [("3+5", 8), ("2+4", 6), ("6*9", 42)])
def test_eval(test_input, expected):
    assert eval(test_input) == expected

But using a fixture to supply the list: [("3+5", 8), ("2+4", 6), ("6*9", 42)].

However, this configuration doesn't work:

@pytest.mark.parametrize("v1, v2, v3", vil_check)
def test_max(v1, v2, v3):
   assert abs(v1) <= 5

The error is that pytest doesn't see vil_check return as iterable. There seems to be a way to use pytest_generate_tests to accomplish this but I'm drawing a blank on how to write it.

You cannot use a fixture in pytest.mark.parametrize, nor can you use it in pytest_generate_tests . Is there a reason why you need a fixture and cannot just use a function here? — MrBean Bremen
– MrBean Bremen, Commented Dec 9, 2021 at 5:29
@MrBeanBremen I need the information that is dynamically created in the function vil_check. This information can will varying which is why I need to parameterize the test so that the number of tests run equals the number of values returned by vil_check. How can I do this using pytest? — Shejo284
– Shejo284, Commented Dec 9, 2021 at 12:56
i still dont get why you need vil_check to be a fixture. Is it like it's being used elsewhere as well? — Shod
– Shod, Commented Dec 9, 2021 at 14:10
@Shod I don't need it to be a fixture. Can I call it from within the test? How then can I populate the list for pytest.mark.parametrize. I'm asking how to do this given the nature of the case study. — Shejo284
– Shejo284, Commented Dec 9, 2021 at 14:54
Just make the fixture a normal function (e.g. remove the decorator) and use it like you do now. — MrBean Bremen
– MrBean Bremen, Commented Dec 9, 2021 at 15:22

Shod · Accepted Answer · 2021-12-09 21:29:16Z

1

As per OP's comment, because vil_check need not be a fixture, here's what you can do - remove the fixture decorator from vil_check and call it in mark.parametrize below:

def vil_check():
   # code
   yield from [(v1,v2,v3), (...), (...)]

@pytest.mark.parametrize("v1,v2,v3", vil_check()):
def test_one(v1,v2,v3):
      assert v1 < 2
      assert v2 > 5
      # code

Few points:

you have spelled parametrized wrong, this may give you error if you have set --strict-markers.
the decorator should not have a :
for performance, i used yield from instead of return in vil_check. This will be efficient in case the list is huge

edited Dec 9, 2021 at 21:29

answered Dec 9, 2021 at 15:20

Shod

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1 Comment

Shejo284 Over a year ago

Thank you! I made the changes per your suggestions and incorporated the points. The test works as I hoped.

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