Multiple param values in fixture

[DavideCanton]

Hi all, I've noticed a not very well documented behavior in parametrizing fixtures.

A fixture with params errors out in the discovery if the params have multiple values. I believe it's because a single name fixture cannot bind different parameter values, but I didn't find anything about it in the docs, neither the error is clear:

import pytest


@pytest.fixture(params=[pytest.param(i, 5) for i in range(5)])
def my_fixture(request):
    return request.param


def test_foo(my_fixture):
    assert my_fixture in {1, 2, 3, 4, 5}

$ pytest tests/foo_test.py

# removed output for brevity

tests/foo_test.py::test_foo: in "parametrize" the number of names (1):
  ['my_fixture']
must be equal to the number of values (2):
  (0, 5)

# removed output for brevity

[portyu9]

Hi @DavideCanton

pytest.param(i, 5) means two parameter values, but your fixture has only one parameter name (my_fixture). That is why pytest reports:

number of names (1) ... must be equal to the number of values (2)

If you want (i, 5) to be the single value of request.param, wrap it as one object:

@pytest.fixture(params=[pytest.param((i, 5)) for i in range(5)])
def my_fixture(request):
    return request.param

You can also omit pytest.param() entirely here:

@pytest.fixture(params=[(i, 5) for i in range(5)])
def my_fixture(request):
    return request.param

Then:

def test_foo(my_fixture):
    i, value = my_fixture

If the 5 was intended to be a test ID rather than another value, use the keyword argument instead:

pytest.param(i, id="5")

Please mark this answer as accepted if it helped, thank you.

[DavideCanton]

I know how does pytest.fixture work, what I mean is that there isn't (or at least there wasn't at the time I opened the question, I didn't recheck) mention in the documentation that you cannot use multiple value parameters in a fixture.

[portyu9]

Thanks for the additional context. A parametrized fixture only has one parameter slot: the fixture itself. Each entry in params becomes the single value exposed as request.param.

So this does not work:

@pytest.fixture(params=[pytest.param(i, 5) for i in range(5)])

because pytest.param(i, 5) creates a parameter set containing two values, while the fixture parametrization expects one.

If the intended fixture value is the pair (i, 5), make that pair the single parameter:

@pytest.fixture(params=[pytest.param((i, 5)) for i in range(5)])
def my_fixture(request):
    return request.param

or simply:

@pytest.fixture(params=[(i, 5) for i in range(5)])
def my_fixture(request):
    return request.param

That said, I agree the documentation is not very explicit about this distinction. pytest.param() is documented as usable with parametrized fixtures, but it does not clearly state that a fixture accepts only one value per parameter set, unlike @pytest.mark.parametrize() where multiple argument names can consume multiple values.

Please mark this answer as accepted if it helped, thank you.