A SQLAlchemy Session requires that an engine be initialized if one isn't
    provided.

def init_session(db_url=None, echo=False, engine=None, settings=None):
    """
    A SQLAlchemy Session requires that an engine be initialized if one isn't
    provided.
    """
    if engine is None:
        engine = init_engine(db_url=db_url, echo=echo, settings=settings)
    return sessionmaker(bind=engine)