Session scoped fixture called twice (another example)

[Sup3rGeo]

Hello!

UPDATE: Environment details:

(venv) D:\pytest-simple-parametrization\otherdir>pip list
Package        Version
-------------- -------
atomicwrites   1.3.0
attrs          19.1.0
colorama       0.4.1
more-itertools 6.0.0
pip            10.0.1
pluggy         0.9.0
py             1.8.0
pytest         4.3.0
setuptools     39.1.0
six            1.12.0

Considering this very simple test case:

from pytest import fixture

@fixture(scope="session", params=[110, 220])
def param1(request):
    return request.param

@fixture(scope="module")
def setup(param1):
    print(f"I am real fixture using param1 = {param1}")

@fixture(scope="session", params=[330, 440])
def param2(request):
    return request.param

def test_normal_operation(setup, param1, param2):
    print(f"Test function using param1 = {param1} and param2 = {param2}")

Running it with --setup-show shows that param2[330] is setup twice, even though it is scoped as session:

test_scope.py
SETUP    S param1[110]
SETUP    S param2[330]
    SETUP    M setup (fixtures used: param1)
        test_scope.py::test_normal_operation[110-330] (fixtures used: param1, param2, setup).
TEARDOWN S param2[330]
SETUP    S param2[440]
        test_scope.py::test_normal_operation[110-440] (fixtures used: param1, param2, setup).
    TEARDOWN M setup
TEARDOWN S param1[110]
SETUP    S param1[220]
    SETUP    M setup (fixtures used: param1)
        test_scope.py::test_normal_operation[220-440] (fixtures used: param1, param2, setup).
TEARDOWN S param2[440]
SETUP    S param2[330]
        test_scope.py::test_normal_operation[220-330] (fixtures used: param1, param2, setup).
    TEARDOWN M setup
TEARDOWN S param2[330]
TEARDOWN S param1[220]

Is it normal or am I missing something? Thanks!

[nicoddemus]

Hi @Sup3rGeo!

Unfortunately that's how the current mechanism works.

One thing that is usually not obvious is that there can be only one instance of a high-level scoped, parametrized fixture at the same type. IOW, if we have created param2[330] for a test invocation, and need param2[440] for the next test, we need to destroy param2[330] in order to create param2[440].

pytest tries to order the tests to minimize fixture setups, but it has to face a tradeoff here, because of how you are parametrizing the tests, either we need to create param1[110] twice or param2[330] twice.

To exemplify:

test_scope.py::test_normal_operation[110-330]
test_scope.py::test_normal_operation[110-440]
test_scope.py::test_normal_operation[220-440]
test_scope.py::test_normal_operation[220-330]

We avoid creating param2[440] twice by reordering the tests, but need to create param2[330] again for the last test. Another possibility would be to order the tests like this:

test_scope.py::test_normal_operation[110-330]
test_scope.py::test_normal_operation[220-330]
test_scope.py::test_normal_operation[220-440]
test_scope.py::test_normal_operation[110-440]

This would create param2[330] and param2[440] only once each, but would require us to create param1[110] twice again.

Hope that clears things up.

#3551 is about improving that order somehow, but this is a complex topic so it still needs some work.

[Sup3rGeo]

Thanks @nicoddemus! Great explanation, will close the question as I now understand the reason why is this happening.