Helper to replace middlewear during tests

When you have middlewear that sets up a database session such as:

@app.middleware("http")
async def db_session_middleware(request: Request, call_next):
    request.state.db = Session()
    response = await call_next(request)
    request.state.db.close()
    return response

It’s handy to be able to replace the middlewear for the duration of test, eg:

@pytest.fixture(scope='session', autouse=True)
def db():
    engine = create_engine(os.environ['TEST_DB_URL'])
    conn = engine.connect()
    transaction = conn.begin()
    try:
        session = Session(bind=conn)
        Base.metadata.create_all(bind=conn, checkfirst=False)

        async def testing_session(request: Request, call_next):
            request.state.db = session
            return await call_next(request)

        with replace_middleware_dispatch(
            app, db_session_middleware, testing_session
        ):
            yield session
    finally:
        transaction.rollback()

@tomchristie - would you welcome a PR that implements replace_middleware_dispatch?

If so, where should I put it in the package?

[tomchristie]

run the database setup

The config docs have an example of how to gear up test database setup. You may want to adapt it so that it runs any migrations.

run each test in its own subtransaction

For test isolation see eg. https://github.com/encode/databases#test-isolation The docs in Starlette have something v. similar to that where they pass rollback_on_shutdown=settings.TESTING to the database middleware.

[cjw296]

How would I change which middlewear is used for a test run versus a production startup?

if I understand the example you link to above, it’s predicated on conftest.py being executed before myproject is ever imported. Is that a documented behaviour of pytest? Feels like it might be an implementation detail that could change in the future…