Database does not update automatically with MySQL and Python

Question

I'm having some trouble updating a row in a MySQL database. Here is the code I'm trying to run:

import MySQLdb

conn=MySQLdb.connect(host="localhost", user="root", passwd="pass", db="dbname")
cursor=conn.cursor()

cursor.execute("UPDATE compinfo SET Co_num=4 WHERE ID=100")
cursor.execute("SELECT Co_num FROM compinfo WHERE ID=100")
results = cursor.fetchall()

for row in results:
    print row[0]

print "Number of rows updated: %d" % cursor.rowcount

cursor.close()
conn.close()

The output I get when I run this program is:

4
Number of rows updated: 1

It seems like it's working but if I query the database from the MySQL command line interface (CLI) I find that it was not updated at all. However, if from the CLI I enter UPDATE compinfo SET Co_num=4 WHERE ID=100; the database is updated as expected.

What is my problem? I'm running Python 2.5.2 with MySQL 5.1.30 on a Windows box.

Answer 1

I am not certain, but I am going to guess you are using a INNODB table, and you haven't done a commit. I believe MySQLdb enable transactions automatically.

Call conn.commit() before calling close.

From the FAQ: Starting with 1.2.0, MySQLdb disables autocommit by default

Answer 2

MySQLdb has autocommit off by default, which may be confusing at first. Your connection exists in its own transaction and you will not be able to see the changes you make from other connections until you commit that transaction.

You can either do conn.commit() after the update statement as others have pointed out, or disable this functionality altogether by setting conn.autocommit(True) right after you create the connection object.

Answer 3

You need to commit changes manually or turn auto-commit on.

The reason SELECT returns the modified (but not persisted) data is because the connection is still in the same transaction.

Answer 4

I've found that Python's connector automatically turns autocommit off, and there doesn't appear to be any way to change this behaviour. Of course you can turn it back on, but then looking at the query logs, it stupidly does two pointless queries after connect to turn autocommit off then back on.