Pytest parametrizing a fixture by multiple yields.

[kdexd]

I need to parametrize a test which requires tmpdir fixture to setup different testcases.

Suppose initial test looks like this:

def test_foo(tmpdir):
    obj1 = SomeObject1(path=tmpdir.strpath, mode='r')
    assert obj1.foo == 'bar0'

    obj2 = SomeObject2(path=tmpdir.strpath, mode='wb')
    assert obj2.foo == 'bar1'

    obj3 = SomeObject3(path=tmpdir.strpath, mode='default')
    assert obj3.foo == 'bar2'

I can't use tmpdir in parametrize, so either I would prefer indirect parametrization, or parametrization through a fixture.

Alternative 1: Indirect Parametrization

@fixture(scope='function')
def testcase_obj(tmpdir, request):
    objs = [SomeObject1(path=tmpdir.strpath, mode='r'),
            SomeObject2(path=tmpdir.strpath, mode='wb'),
            SomeObject3(path=tmpdir.strpath, mode='default')]
    return objs[request.param]

@parametrize(
    'testcase_obj, expected_foo',
    [(0, 'bar0'), (1, 'bar1'), (2, 'bar2')],
    indirect=['testcase_obj']
)
def test_foo(testcase_obj, expected_foo):
    assert testcase_obj.foo == expected_foo

Alternative 2: Parametrizing through fixture

@fixture(scope='function', params=range(3))
def testcase(tmpdir, request):
    objs = [SomeObject1(path=tmpdir.strpath, mode='r'),
            SomeObject2(path=tmpdir.strpath, mode='wb'),
            SomeObject3(path=tmpdir.strpath, mode='default')]
    expected_foos = ['bar0', 'bar1', 'bar2']
    return objs[request.param], expected_foos[request.param]

def test_foo(testcase):
    testcase_obj, expected_foo = testcase
    assert testcase_obj.foo == expected_foo

Alternative two is a little less entangled but one can easily miss a bug if he adds testcases in function body but forgets to update range(3).

Currently fixtures yield only once but it will be really great if multiple yields results in parametrization.
Example code would simply reduce to:

@fixture(scope='function')
def testcase(tmpdir, request):
    objs = [SomeObject1(path=tmpdir.strpath, mode='r'),
            SomeObject2(path=tmpdir.strpath, mode='wb'),
            SomeObject3(path=tmpdir.strpath, mode='default')]
    expected_foos = ['bar0', 'bar1', 'bar2']

    for obj, expected_foo in zip(objs, expected_foos):
        yield obj, expected_foo

def test_foo(testcase):
    testcase_obj, expected_foo = testcase
    assert testcase_obj.foo == expected_foo

[nicoddemus]

Hi @karandesai-96!

Please refer to the discussion in #1595, but the gist is this:

... we have to obtain all items during the collection phase, and fixtures parametrized that way won't be executed until the first test that uses it executes, long past the collection phase.

I'm closing this for now, but feel free to ask for clarification!