What's the right way to mock on_startup callbacks?

[dan-jacobson]

When I'm trying to test my app, I have an on_startup callback to connect to a DB. It looks roughly like the example.

However, how can I test it? There's times I want to run my tests without the actual DB connection, and so I try to mock it out. The best I can come up with is:

@pytest.fixture(scope="session")
async def test_client():
    mock_engine = MagicMock()

    def mock_get_db_connection(app):
        app.state.engine = mock_engine
        return mock_engine

    get_db_connection_idx = app.on_startup.index(get_db_connection)
    app.on_startup[get_db_connection_idx] = mock_get_db_connection

    async with AsyncTestClient(app=app) as client:
        yield client

This kinda feels hacky. Is there a better way to just directly mock or patch the get_db_connection() call? I tried all the variants of patch("app.app.on_startup") and patch("app.app.get_db_connection") I could think of.

How do other folks handle this?