nested looping of a test inside of a scenario produces unexpected execution order

[spleshakov]

I need to get the following order of tests

test_1.py::TestAB::test_1[a1] PASSED
test_1.py::TestAB::test_2[a1] PASSED
test_1.py::TestAB::test_3[b1-a1] PASSED
test_1.py::TestAB::test_3[b2-a1] PASSED
test_1.py::TestAB::test_3[b3-a1] PASSED

test_1.py::TestAB::test_1[a2] PASSED
test_1.py::TestAB::test_2[a2] PASSED
test_1.py::TestAB::test_3[b1-a2] PASSED
test_1.py::TestAB::test_3[b2-a2] PASSED
test_1.py::TestAB::test_3[b3-a2] PASSED

test_1.py::TestAB::test_1[a3] PASSED
test_1.py::TestAB::test_2[a3] PASSED
test_1.py::TestAB::test_3[b1-a3] PASSED
test_1.py::TestAB::test_3[b2-a3] PASSED
test_1.py::TestAB::test_3[b3-a3] PASSED

Given that

@pytest.mark.parametrize("a", ['a1', 'a2', 'a3'], scope="class")
class TestAB:

    def test_1(self, a):
        pass

    def test_2(self, a):
        pass

    def test_3(self, a):
        pass

Produces

test_1.py::TestAB::test_1[a1] PASSED
test_1.py::TestAB::test_2[a1] PASSED
test_1.py::TestAB::test_3[a1] PASSED
test_1.py::TestAB::test_1[a2] PASSED
test_1.py::TestAB::test_2[a2] PASSED
test_1.py::TestAB::test_3[a2] PASSED
test_1.py::TestAB::test_1[a3] PASSED
test_1.py::TestAB::test_2[a3] PASSED
test_1.py::TestAB::test_3[a3] PASSED

and

@pytest.mark.parametrize("b", ["b1", "b2", "b3"])
def test_3(b):
    pass

produces

test_1.py::test_3[b1] PASSED
test_1.py::test_3[b2] PASSED
test_1.py::test_3[b3] PASSED

I would expect that

@pytest.mark.parametrize("a", ['a1', 'a2', 'a3'], scope="class")
class TestAB:

    def test_1(self, a):
        pass

    def test_2(self, a):
        pass

    @pytest.mark.parametrize("b", ["b1", "b2", "b3"])
    def test_3(self, a, b):
        pass

should do the work... But instead I get

test_1.py::TestAB::test_1[a1] PASSED
test_1.py::TestAB::test_2[a1] PASSED
test_1.py::TestAB::test_3[b1-a1] PASSED
test_1.py::TestAB::test_1[a2] PASSED
test_1.py::TestAB::test_2[a2] PASSED
test_1.py::TestAB::test_3[b1-a2] PASSED
test_1.py::TestAB::test_1[a3] PASSED
test_1.py::TestAB::test_2[a3] PASSED
test_1.py::TestAB::test_3[b1-a3] PASSED
test_1.py::TestAB::test_3[b2-a1] PASSED
test_1.py::TestAB::test_3[b2-a2] PASSED
test_1.py::TestAB::test_3[b2-a3] PASSED
test_1.py::TestAB::test_3[b3-a1] PASSED
test_1.py::TestAB::test_3[b3-a2] PASSED
test_1.py::TestAB::test_3[b3-a3] PASSED

Why is this happening and how to solve it?

Thanks!