This is How I implement the DB connection (PostgreSQL):

class DBContext(object):
    def __init__(self, session=None):
        self._engine = create_engine(POSTGRES_URL)
        self._session: Session = session or scoped_session(sessionmaker(
            autocommit=False, autoflush=False, bind=self._engine))
        self.extrnal_session = True if session else False
        self.current_session: Session = None

    def start_session(self):
        self.current_session = self._session

    def begin_transaction(self):
        try:
            self.current_session.begin()
        except:
            pass

    def commit(self):
        if not self.extrnal_session:
            self.current_session.commit()

    def end_session(self):
        if self.current_session and not self.extrnal_session:
            self.current_session.close()

    @contextmanager
    def db_session(self):
        try:
            self.start_session()
            yield self.current_session
        finally:
            self.end_session()

And in one of the endpoints in my FastAPI app I have something like this:

@router.get(
    "/user/{uuid}/item/",
    # dependencies=[Depends(auth)],
    response_model=GuestResponseSerializer,
    responses={
        404: {"model": ResponseSerializer}
    }
)
def get_user(uuid: UUID):
    retries = 0
    row = None
    while retries < 10:
        print("try number", retries, uuid)
        db = DBContext()
        db.start_session()
        query = db.current_session.query(Guest)
        query = query.filter(Guest.uuid == uuid)
        row = query.first()
        if row:
            break
        retries += 1
    if not row:
        return JSONResponse({"message": "not found"}, status_code=404)

    guest = GuestEntity.from_orm(row)
    return guest

It was so weird. It does not fetch the user with the UUID the first time and it gets in the while loop and sometimes it takes 5 times to get the same row. I don’t know how it occurs!! And how to fix it.
Output logs:

try number 0 c0813595-7328-4f94-9951-767792553496
try number 1 c0813595-7328-4f94-9951-767792553496
try number 2 c0813595-7328-4f94-9951-767792553496
try number 3 c0813595-7328-4f94-9951-767792553496
try number 4 c0813595-7328-4f94-9951-767792553496
try number 5 c0813595-7328-4f94-9951-767792553496
try number 6 c0813595-7328-4f94-9951-767792553496
try number 7 c0813595-7328-4f94-9951-767792553496
try number 8 c0813595-7328-4f94-9951-767792553496
try number 9 c0813595-7328-4f94-9951-767792553496
INFO:     172.18.0.13:36090 - "GET /v1/user/c0813595-7328-4f94-9951-767792553496/item/ HTTP/1.0" 200 OK