How to parametrize a Pytest fixture

Question

Consider the following Pytest:

import pytest

class TimeLine(object):
    instances = [0, 1, 2]

@pytest.fixture
def timeline():
    return TimeLine()

def test_timeline(timeline):
    for instance in timeline.instances:
        assert instance % 2 == 0

if __name__ == "__main__":
    pytest.main([__file__])

The test test_timeline uses a Pytest fixture, timeline, which itself has the attribute instances. This attribute is iterated over in the test, so that the test only passes if the assertion holds for every instance in timeline.instances.

What I actually would like to do, however, is to generate 3 tests, 2 of which should pass and 1 of which would fail. I've tried

@pytest.mark.parametrize("instance", timeline.instances)
def test_timeline(timeline):
    assert instance % 2 == 0

but this leads to

AttributeError: 'function' object has no attribute 'instances'

As I understand it, in Pytest fixtures the function 'becomes' its return value, but this seems to not have happened yet at the time the test is parametrized. How can I set up the test in the desired fashion?

Answer 1

This is actually possible via indirect parametrization.

This example does what you want with pytest 3.1.2:

import pytest

class TimeLine:
    def __init__(self, instances):
        self.instances = instances

@pytest.fixture
def timeline(request):
    return TimeLine(request.param)

@pytest.mark.parametrize(
    'timeline',
    ([1, 2, 3], [2, 4, 6], [6, 8, 10]),
    indirect=True
)
def test_timeline(timeline):
    for instance in timeline.instances:
        assert instance % 2 == 0

if __name__ == "__main__":
    pytest.main([__file__])

Also see this similar question.