Parameterize fixture depending on other fixture?

[HiPhish]

Hello,

I want to know if there is a way to parameterize a fixture not depending on some known value, but depending on another fixture. I know that I can parameterize fixtures like this:

@pytest.fixture(params=[[1, 2, 3], [7, 8, 9]])
def numbers(request):
    return request.param

# This test will run twice, once per numbers list
def test_list_not_empty(numbers):
    assert len(numbers) > 0

I also know that fixtures can depend on parameterized fixtures:

@pytest.fixture(params=[[1, 2, 3], [7, 8, 9]])
def numbers(request):
    return request.param


# This fixture will be requested twice because `numbers` has two parameters
@pytest.fixture
def first_number(numbers):
    return numbers[0]


def test_first_in_list(first_number, numbers)
    assert first_number in numbers

What I want to do is combine both:

@pytest.fixture(params=[[1, 2, 3], [7, 8, 9]])
def numbers(request):
    return request.param

@pytest.fixture
def number(numbers):
    for n in numbers:
        yield n  # <-- this is wrong, yield is for fixtures that need cleanup

def test_contains(number, numbers)
    assert number in numbers

The above code is of course wrong because fixtures are only supposed to yield once and yielding is used for cleanup, but I hope it illustrates the idea that I want to call test_contains with arguments (1, [1,2,3]), (2, [1,2,3]), (3, [1,2,3]), (7, [7,8,9]) and so on.

This is of course a silly toy example, but I do have real use cases where one fixture drives other fixtures and some of those other fixtures branch out to provide even more values. For example, let's say I have a number of XML Schema files and modules of classes. For each class I want to verify that it matches the definition from the schema. I have the following fixtures:

• module is parameterized and will provide each of the modules one at a time; this is the top-most fixture
• schema depends on module and provides the parsed XML Schema tree; since for each module there is only one schema file there is no need to parameterize anything
• klass depends on module and should provide one class at a time from the module, so it needs to be parametrized on module

Then the test can look something like this:

def test_class_defined(klass, schema):
    assert ...

Without being able to parameterize klass I have to resort to assertions in a for-loop:

def test_classes_defined(module, schema):
    for klass in get_classes(module):
        assert ...

This is bad because the test will fail on the very first failure, so I won't know if there is only one broken class or a hundred broken classes. Is there a way to achieve what I'm looking for? I don't mind if I have to do it in a roundabout way, all I want is to run the test once per class per schema.

[RonnyPfannschmidt]

As currently fixture values happen after collecting there's no direct mechanism to do so

A crude workaround would be to collect the data at collection time and using the generate tests hook to create fitting test items

[HiPhish]

I see, that's a shame, it would be really useful. I have found out about the new subtests feature. It's not quite the same, but it does come close. Still, I think my idea would be better. Is there a technical reason why it cannot be done, or is it just a case of "no one has yet thought of it"?

A crude workaround would be to collect the data at collection time and using the generate tests hook to create fitting test items

Can you please point me to the relevant page or pages in the documentation?

[RonnyPfannschmidt]

Its been asked before

Making it worknis the tricky thing

[HiPhish]

I see, that's unfortunate. Thank you for the clarification.