I'm trying to execute a basic `INSERT` statement on a MySQL table from a Python script using MySQLdb. My table looks like this:

```
CREATE TABLE `testtable` (
    `id` int(11) NOT NULL AUTO_INCREMENT,
    `testfield` varchar(255) NOT NULL,
    PRIMARY KEY (`id`)
)

```

Running this query from the MySQL command line works fine:

```
INSERT INTO `testtable` (`id`, `testfield`) VALUES (NULL, 'testvalue');

```

But when I try to execute the query from a Python script, no rows get inserted. Here's my code:

```
conn = MySQLdb.connect(host=db_host, port=db_port, user=db_user, passwd=db_password, db=db_database)
cursor = conn.cursor ()
cursor.execute ("INSERT INTO `testtable` (`id`, `testfield`) VALUES (NULL, 'testvalue')")
print "Number of rows inserted: %d" % cursor.rowcount
cursor.close()
conn.close()

```

Oddly, this will print "Number of rows inserted: 1." I can also confirm that this query increments the ID field, because when I add another row via the command line, the value of its ID is the same as if the Python script had successfully inserted its rows. However, running a `SELECT` query returns none of the rows from the script.

Any idea what's going wrong?