pytest parameterized ids function differences between fixtures and functions/classes

output session

(venv) $ pytest -v test_ids.py 
============================= test session starts ==============================
platform darwin -- Python 3.5.2, pytest-3.0.0, py-1.4.31, pluggy-0.3.1 -- /Users/okken/projects/book/bopytest/Book/venv/bin/python3.5
cachedir: ../.cache
rootdir: /Users/okken/projects/book/bopytest/Book/code/pytest/um_project, inifile: 
collected 8 items 

test_ids.py::test_ids_strings[a] PASSED
test_ids.py::test_ids_strings[b] PASSED
test_ids.py::test_ids_paramterized_function[<1>-<2>] PASSED
test_ids.py::test_ids_paramterized_function[<2>-<4>] PASSED
test_ids.py::TestIds::test_ids_paramterized_class[<1>-<2>] PASSED
test_ids.py::TestIds::test_ids_paramterized_class[<2>-<4>] PASSED
test_ids.py::test_ids_parameterized_fixture[<(1, 2)>] PASSED
test_ids.py::test_ids_parameterized_fixture[<(2, 4)>] PASSED

test_ids.py

# -----------------------------------------------------------------------
# To Demonstrate ids work differently in functions/classes than fixtures
# -----------------------------------------------------------------------

def id_func(value):
    return '<{}>'.format(value)

@pytest.mark.parametrize("x,expected", testdata, ids=id_func)
def test_ids_paramterized_function(x,expected):
    assert times_2(x) == expected

@pytest.mark.parametrize("x,expected", testdata, ids=id_func)
class TestIds:
    def test_ids_paramterized_class(self,x,expected):
        assert times_2(x) == expected

@pytest.fixture(params=testdata, ids=id_func)
def x_expected_data(request):
    x, expected = request.param
    return {'x': x, 'expected':expected }

def test_ids_parameterized_fixture(x_expected_data):
    x = x_expected_data['x']
    expected = x_expected_data['expected']
    assert times_2(x) == expected