{"id":203,"date":"2015-05-08T17:00:23","date_gmt":"2015-05-08T21:00:23","guid":{"rendered":"http:\/\/pfsensesetup.com\/pythonscript.net\/?p=203"},"modified":"2015-05-08T14:42:04","modified_gmt":"2015-05-08T18:42:04","slug":"python-database-programming-part-nine","status":"publish","type":"post","link":"http:\/\/pfsensesetup.com\/pythonscript.net\/python-database-programming-part-nine\/","title":{"rendered":"Python Database Programming: Part Nine"},"content":{"rendered":"

\"Python<\/a>Python Database Programming: Committing and Rolling Back Transactions<\/strong><\/span><\/p>\n

Each connection, while it is engaged in action, manages a transaction. With SQL, data is not modified unless you commit a transaction. The database then guarantees that it will perform all of the modifications in the transaction or none. As a result, you will not leave your database in a potentially erroneous condition.<\/p>\n

To commit a transaction, we call the commit method of a connection:<\/p>\n

conn.commit()<\/strong><\/p>\n

Note the the transaction methods are part of the connection class, not the cursor class.<\/p>\n

If something goes wrong, like an exception is thrown that you can handle, you should call the rollback method to undo the effects of the incomplete transaction; this will restore the database to the state it was in before you started the transaction, guaranteed:<\/p>\n

conn.rollback()<\/strong><\/p>\n

The capability to roll back a transaction is very important, because you can handle errors by ensuring that the database does not get changed. In addition, rollbacks are very useful for testing. You can insert, modify and delete a number of rows as part of a unit test and then roll back the transaction to undo the effects of all the changes. This enables your unit tests to run without making any permanent changes to the database. It also enables your unit tests to be run repeatedly, because each run resets the data.<\/p>\n

The DB API defines several globals that need to be defined at the module level. You can use these globals to determine information about the database module and the features it supports. The following table lists these globals.<\/p>\n\n\n\n\n\n
Global<\/th>\nWhat It Holds<\/th>\n<\/tr>\n
Apilevel<\/td>\nShould hold ‘2.0’ for the DB API 2.0, or ‘1.0’ for the 1.0 API.<\/td>\n<\/tr>\n
Paramstyle<\/td>\nDefines how you can indicate the placeholders for dynamic data in your SQL statements. The values include:<\/p>\n