InternalError：当前事务被中止，命令被忽略，直到事务块结束（受UNIQUE约束）

Question

I tried to get executed with my except: statement... while attempt to oppose the functionality of UNIQUE constraint..But ended with exceptional error.. The Postgresql database table already contains the row that I have used in

db.insert("The News","AparnaKumar",1995,234569654)

but it works well on inserting unrepeated rows..

import psycopg2
class database:

    def __init__(self):

        self.con=psycopg2.connect("dbname='book_store' user='postgres' password='5283' host='localhost' port='5432' ")
        self.cur=self.con.cursor()
        self.cur.execute("CREATE TABLE if not exists books(id SERIAL PRIMARY KEY,title TEXT NOT NULL UNIQUE,author TEXT NOT NULL,year integer NOT NULL,isbn integer NOT NULL UNIQUE)")
        self.con.commit()

    def insert(self,title,author,year,isbn):
      try:
        self.cur.execute("INSERT INTO books(title,author,year,isbn) VALUES(%s,%s,%s,%s)",(title,author,year,isbn))
        self.con.commit()
      except:
          print("already exists..")

    def view(self):
        self.cur.execute("SELECT * FROM books")
        rows=self.cur.fetchall()
        print(rows)

    def search(self,title=None,author=None,year=None,isbn=None):
        self.cur.execute("SELECT * FROM books WHERE title=%s or author=%s or year=%s or isbn=%s",(title,author,year,isbn))
        row=self.cur.fetchall()
        print(row)

db=database()
db.insert("The News","AparnaKumar",1995,234569654)
db.view()
db.search(year=1995)

Answer 1

You can either modify your python function to rollback the transaction, or modify the sql to not insert a new row if a conflict occurs (postgresql versions 9.5+):

option 1:

def insert(self,title,author,year,isbn):
  try:
    self.cur.execute("INSERT INTO books(title,author,year,isbn) VALUES(%s,%s,%s,%s)",(title,author,year,isbn))
    self.con.commit()
  except:
    self.con.rollback()
      print("already exists..")

option 2 (works for postgresql versions 9.5 or later):

def insert(self,title,author,year,isbn):
  try:
    self.cur.execute("INSERT INTO books(title,author,year,isbn) VALUES(%s,%s,%s,%s) ON CONFLICT DO NOTHING",(title,author,year,isbn))
    self.con.commit()
  except:
      print("already exists..")

Answer 2

Use a SAVEPOINT .

See the second question in the psycopg FAQ list .

Answer 3

If you want to keep an transaction open, you should use a SAVEPOINT and ROLLBACK to that SAVEPOINT when the exception occurs. But since you use a catchall exception (too broad), this will also happen on other errors. So it gets a bit out of control.

Perhaps a very simple solution is to check using a SELECT if the record you're about to insert already exists.