在开启事务时，使用SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;语句来设置事务的隔离级别为串行化隔离级别，该隔离级别最大程度地保障并发事务的正确性，但付出了相应的额外开销。

示例代码：

import psycopg2

conn = psycopg2.connect("dbname=test user=postgres password=123456 host=localhost port=5432")
cur = conn.cursor()

# 开启事务，并设置隔离级别为串行化隔离级别
cur.execute("SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;")
cur.execute("BEGIN;")

try:
    cur.execute("UPDATE account SET balance = balance - 100 WHERE id = 1;")
    cur.execute("UPDATE account SET balance = balance + 100 WHERE id = 2;")
except Exception as e:
    print(e)
    conn.rollback()

conn.commit()