conftest.py

@pytest.fixture
def test_shadow_page(inline, shadow_content):
    return inline("""
        <custom-shadow-element></custom-shadow-element>
        <script>
            customElements.define('custom-shadow-element',
                class extends HTMLElement {{
                    constructor() {{
                            super();
                            this.attachShadow({{mode: 'open'}}).innerHTML = `
                                {{ {0} }}
                            `;
                        }}
                }});
        </script>""".format(shadow_content))

find.py

@pytest.mark.parametrize("using,value",
                         [("css selector", "#linkText"),
                          ("link text", "full link text"),
                          ("partial link text", "link text"),
                          ("tag name", "a"),
                          ("xpath", "//a")])
def test_find_element(session, test_shadow_page, using, value):
    # Step 8 - 9
    session.url = test_shadow_page("<div><a href=# id=linkText>full link text</a></div>")
    custom_element = session.find.css("custom-shadow-element", all=False)
    shadow_root = custom_element.shadow_root
    response = find_element(session, shadow_root, using, value)
    assert_success(response)