psycopg2/examples/cursor.py

# cursor.py - how to subclass the cursor type
#
# Copyright (C) 2004 Federico Di Gregorio  <fog@debian.org>
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by the
# Free Software Foundation; either version 2, or (at your option) any later
# version.
#
# This program is distributed in the hope that it will be useful, but
# WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY
# or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
# for more details.

## put in DSN your DSN string

DSN = 'dbname=test'

## don't modify anything below this line (except for experimenting)

import sys
import psycopg
import psycopg.extensions

if len(sys.argv) > 1:
    DSN = sys.argv[1]

print "Opening connection using dsn:", DSN

conn = psycopg.connect(DSN)
print "Encoding for this connection is", conn.encoding


class NoDataError(psycopg.ProgrammingError):
    """Exception that will be raised by our cursor."""
    pass

class Cursor(psycopg.extensions.cursor):
    """A custom cursor."""

    def fetchone(self):
        """Like fetchone but raise an exception if no data is available.

        Note that to have .fetchmany() and .fetchall() to raise the same
        exception we'll have to override them too; even if internally psycopg
        uses the same function to fetch rows, the code path from Python is
        different.
        """
        d = psycopg.extensions.cursor.fetchone(self)
        if d is None:
	    raise NoDataError("no more data")
        return d
    
curs = conn.cursor(factory=Cursor)
curs.execute("SELECT 1 AS foo")
print "Result of fetchone():", curs.fetchone()

# now let's raise the exception
try:
    curs.fetchone()
except NoDataError, err:
    print "Exception caugth:", err  

conn.rollback()