How to enable MySQL client auto re-connect with MySQLdb?

Question

I came across PHP way of doing the trick:

my_bool reconnect = 1;
mysql_options(&mysql, MYSQL_OPT_RECONNECT, &reconnect);

but no luck with MySQLdb (python-mysql).

Can anybody please give a clue? Thanks.

Answer 1

I solved this problem by creating a function that wraps the cursor.execute() method since that's what was throwing the MySQLdb.OperationalError exception. The other example above implies that it is the conn.cursor() method that throws this exception.

import MySQLdb

class DB:
  conn = None

  def connect(self):
    self.conn = MySQLdb.connect()

  def query(self, sql):
    try:
      cursor = self.conn.cursor()
      cursor.execute(sql)
    except (AttributeError, MySQLdb.OperationalError):
      self.connect()
      cursor = self.conn.cursor()
      cursor.execute(sql)
    return cursor

db = DB()
sql = "SELECT * FROM foo"
cur = db.query(sql)
# wait a long time for the Mysql connection to timeout
cur = db.query(sql)
# still works

Answer 2

I had problems with the proposed solution because it didn't catch the exception. I am not sure why.

I have solved the problem with the ping(True) statement which I think is neater:

import MySQLdb
con=MySQLdb.Connect()
con.ping(True)
cur=con.cursor()

Got it from here: http://www.neotitans.com/resources/python/mysql-python-connection-error-2006.html

Answer 3

If you are using ubuntu Linux there was a patch added to the python-mysql package that added the ability to set that same MYSQL_OPT_RECONNECT option (see here). I have not tried it though.

Unfortunately, the patch was later removed due to a conflict with autoconnect and transations (described here).

The comments from that page say: 1.2.2-7 Published in intrepid-release on 2008-06-19

python-mysqldb (1.2.2-7) unstable; urgency=low

[ Sandro Tosi ] * debian/control - list items lines in description starts with 2 space, to avoid reformat on webpages (Closes: #480341)

[ Bernd Zeimetz ] * debian/patches/02_reconnect.dpatch: - Dropping patch: Comment in Storm which explains the problem:

    # Here is another sad story about bad transactional behavior. MySQL
    # offers a feature to automatically reconnect dropped connections.
    # What sounds like a dream, is actually a nightmare for anyone who
    # is dealing with transactions. When a reconnection happens, the
    # currently running transaction is transparently rolled back, and
    # everything that was being done is lost, without notice. Not only
    # that, but the connection may be put back in AUTOCOMMIT mode, even
    # when that's not the default MySQLdb behavior. The MySQL developers
    # quickly understood that this is a terrible idea, and removed the
    # behavior in MySQL 5.0.3. Unfortunately, Debian and Ubuntu still
    # have a patch right now which *reenables* that behavior by default
    # even past version 5.0.3.

Answer 4

I needed a solution that works similarly to Garret's, but for cursor.execute(), as I want to let MySQLdb handle all escaping duties for me. The wrapper module ended up looking like this (usage below):

#!/usr/bin/env python

import MySQLdb

class DisconnectSafeCursor(object):
    db = None
    cursor = None

    def __init__(self, db, cursor):
        self.db = db
        self.cursor = cursor

    def close(self):
        self.cursor.close()

    def execute(self, *args, **kwargs):
        try:
            return self.cursor.execute(*args, **kwargs)
        except MySQLdb.OperationalError:
            self.db.reconnect()
            self.cursor = self.db.cursor()
            return self.cursor.execute(*args, **kwargs)

    def fetchone(self):
        return self.cursor.fetchone()

    def fetchall(self):
        return self.cursor.fetchall()

class DisconnectSafeConnection(object):
    connect_args = None
    connect_kwargs = None
    conn = None

    def __init__(self, *args, **kwargs):
        self.connect_args = args
        self.connect_kwargs = kwargs
        self.reconnect()

    def reconnect(self):
        self.conn = MySQLdb.connect(*self.connect_args, **self.connect_kwargs)

    def cursor(self, *args, **kwargs):
        cur = self.conn.cursor(*args, **kwargs)
        return DisconnectSafeCursor(self, cur)

    def commit(self):
        self.conn.commit()

    def rollback(self):
        self.conn.rollback()

disconnectSafeConnect = DisconnectSafeConnection

Using it is trivial, only the initial connect differs. Extend the classes with wrapper methods as per your MySQLdb needs.

import mydb

db = mydb.disconnectSafeConnect()
# ... use as a regular MySQLdb.connections.Connection object

cursor = db.cursor()

# no more "2006: MySQL server has gone away" exceptions now
cursor.execute("SELECT * FROM foo WHERE bar=%s", ("baz",))