If an SQLalchemy session is doing a read/write to a table, can a different session do operations to this table?

Question

I am building an app where my database updates a new live stock price every second. A customer can choose to buy the stock, and this order will be saved to the database.

I am currently doing basic read/write operations using Sqlalchemy in a fastapi app, as shown in the example below. When i deploy the application, i tried triggering this function at the same time on 2 devices. (Click on buy stock at the same time) The result is that it will fail on one device, which is the behaviour that i desire, as for each timestamp, only 1 order should be allowed to occur.

I am curious as to whether this behaviour is due to the session by Sqlalchemy, and whether i am right in saying that when 1 session is occuring, another session cannot occur. I read online that it is still possible for postgres database to have race conditions if not handled correctly. Does using sessions automatically deal with race conditions?

       with self.Session.begin() as session:
            n_rows = session.query(Order).count()
            if n_rows >= 1000:
                oldest_data = session.query(Order).first()
                session.delete(oldest_data)
            order_model = Order(**order.__dict__)
            session.add(order_model)
            session.commit()