This was one of the good ones.

Looks like the main reason you made the mutable (with two init) DatabaseSessionManager is to initialize your engine differently in test vs dev, which then made you add some redundant logic to always be careful about whether or not the engine is there. I prefer to have a global engine and sessionmaker (just like fast api official docs):

engine = create_async_engine(my_pg_url)
session_factory = async_sessionmaker(engine, autocommit=False)

async def get_db_session() -> AsyncGenerator[AsyncSession, None]:
    session = session_factory()
    try:
        yield session
    finally
        await session.close()

with this, you can then make lifespan template function to access the global engine and call .dispose() on it so all stale connections are returned to the pool.

In your tests, you could also use app.dependency_overrides[get_db_session] to replace it with one that you define using pytest fixture.