Can I create a pytest.fixture that returns combinations of arguments?

Question

The inputs to the the algorithm being tested is a list of strings (known in advance) with associated years, e.g. all of the following are valid inputs:

['A_2018', 'B_2019', 'C_2018']
[]
['A_2018']

I've got the following simple year fixture:

@pytest.fixture(params=range(2018, 2025))
def year(request):
    return request.param

If I create individual fixtures for each valid string:

@pytest.fixture(params=['A', ''])
def A(request, year):
    return request.param, year

@pytest.fixture(params=['B', ''])
def B(request, year):
    return request.param, year

etc. and use them in tests like:

def test_foo(A, B):
    args = []
    if A[0]:
        args.append('%s_%d' % A)
    if B[0]:
        args.append('%s_%d' % B)
    assert old_algorithm(args) == new_algorithm(args)

I get

['A_2018', 'B_2018']
['A_2019', 'B_2019']
['A_2020', 'B_2020']

etc. where the year is always the same for both parameters.

Is there a way to create all combinations?

Answer 1

Is a fixture generating combinations a requirement? Because otherwise, applying pytest.mark.parametrize separately for each test input arg generates the input args combinations just fine. In the example below both fixtures A and B are parametrized separately, generating (2025 - 2018)**2 tests in total:

@pytest.fixture
def A(request):
    return 'A', request.param


@pytest.fixture
def B(request):
    return 'B', request.param


@pytest.mark.parametrize('A', range(2018, 2025), indirect=True, ids=lambda year: 'A({})'.format(year))
@pytest.mark.parametrize('B', range(2018, 2025), indirect=True, ids=lambda year: 'B({})'.format(year))
def test_foo(A, B):
    assert A[0] == 'A' and B[0] == 'B'

As a result, 49 tests are produced:

$ pytest --collect-only | grep collected
collected 49 items