Graceful Primary Key Error handling in Python/psyc

问题:

Using Python 2.7 and

In [150]: psycopg2.version Out[150]: '2.4.2 (dt dec pq3 ext)'

I have a simple python scripts that processing transactions and writes data to a database. Occasionally there is an insert that violates my primary key. This is fine, i just want it to ignore that record and continue on it merry way. The problem I am having is that psycopg2 primary key error is aborting the entire transaction block and all inserts after the error fail. Here is an example error

ERROR: duplicate key value violates unique constraint "encounter_id_pkey"
DETAIL:  Key (encounter_id)=(9012235) already exists.

This is on the next insert. not a violation.

Inserting: 0163168~9024065
ERROR: current transaction is aborted, commands ignored until end of transaction block

The Second error repeats itself for every insert. Here is a simplified loop. I am looping through a pandas data frame, but it could be any loop.

conn = psycopg2.connect("dbname='XXXX' user='XXXXX' host='XXXX' password='XXXXX'")

cur = conn.cursor()

for i, val in df2.iteritems():
    try:
        cur = conn.cursor()
        cur.execute("""insert into encounter_id_table (
        encounter_id,current_date  )
        values       
        (%(create_date)s, %(encounter_id)s ) ;""",
        'encounter_id':i.split('~')[1],  
        'create_date': datetime.date.today() })           
        cur.commit()
        cur.close() 
    except Exception , e:
        print 'ERROR:', e[0]
        cur.close()
 conn.close()   

Again the basic idea is to gracefully handle the Error. In the dictum of Admiral Nelson of the Royal Navy: "Damn the maneuvers go straight at them". Or in our case damn the Errors go straight at them." I thought by opening a cursor on every insert that I would be resetting the transaction block. I do not want to have to reset the connection just because of a primary key error. Is there something i am just missing?

Thanks before hand for your time.

John

回答1:

You should rollback transaction on error.

I've added one more try..except..else construction in the code bellow to show the exact place where exception will occur.

try:
    cur = conn.cursor()

    try:
        cur.execute("""insert into encounter_id_table (
            encounter_id,current_date  )
            values       
            (%(create_date)s, %(encounter_id)s ) ;""",
            'encounter_id':i.split('~')[1],  
            'create_date': datetime.date.today() })
    except psycopg2.IntegrityError:
        conn.rollback()
    else:
        conn.commit()

    cur.close() 
except Exception , e:
    print 'ERROR:', e[0]

回答2:

First of all: CURRENT_DATE is a reserved word in every SQL standard as well as in PostgreSQL. You cannot use it as identifier without double-quoting it. I would strongly advice not to use it at all. I renamed the column to curdate in my example

Next, I am no expert in python syntax, but you seem to have reversed the order of your insert-columns:

(%(create_date)s, %(encounter_id)s )

Should be:

( %(encounter_id)s, %(create_date)s)

To your main question: you can avoid the problem altogether by checking if the key is already in the table before using it in the insert command:

INSERT INTO encounter_id_table (encounter_id, curdate)
SELECT 1234, now()::date
WHERE  NOT EXISTS (SELECT * FROM encounter_id_table t
                   WHERE t.encounter_id = 1234);

In Python syntax, that should be:

cur.execute("""INSERT INTO encounter_id_table (encounter_id, curdate)
    SELECT %(encounter_id)s, %(create_date)s,
    WHERE  NOT EXISTS (
           SELECT * FROM encounter_id_table t
           WHERE t.encounter_id = %(encounter_id)s);""",
  {'encounter_id':i.split('~')[1],  
  'create_date': datetime.date.today()})