'Test transforming of values.'

@pytest.mark.parametrize('val,    expected', [('', None), ('1,2,3,4', (1, 2, 3, 4))])
def test_transform(self, klass, val, expected):

    transformed = klass().transform(val)
    assert (transformed == expected)
    if (expected is not None):
        assert (transformed.top == expected[0])
        assert (transformed.bottom == expected[1])
        assert (transformed.left == expected[2])
        assert (transformed.right == expected[3])
