Eu dou mock no session do banco de dados, estou usando Pytest. Vai precisar de um banco de dados para teste, mas achei essa a melhor abordagem.

A fixture fica assim:

from sqlalchemy.orm import declarative_base
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker

Base = declarative_base()

@pytest.fixture
def client(session):
    def get_session_override():
        return session
    app.dependency_overrides[get_session] = get_session_override
    with TestClient(app) as client:
        yield client
    app.dependency_overrides.clear()

@pytest.fixture
def session():
    engine = create_engine(settings.DATABASE_URL_TEST)
    Session = sessionmaker(bind=engine)
    Base.metadata.drop_all(engine)
    Base.metadata.create_all(engine)
    session = Session()
    yield session
    session.close()
    Base.metadata.drop_all(engine)