Simon Willison’s Weblog

Datasette 1.0a10. The only changes in this alpha release concern the way Datasette handles database transactions. The database.execute_write_fn() internal method used to leave functions to implement transactions on their own—it now defaults to wrapping them in a transaction unless they opt out with the new transaction=False parameter.

In implementing this I found several places inside Datasette—in particular parts of the JSON write API—which had not been handling transactions correctly. Those are all now fixed. # 18th February 2024, 5:10 am

How to implement a “dry run mode” for data imports in Django (via) Adam Johnson describes in detail a beautiful pattern for implementing a dry-run mode for a Django management command, by executing ORM calls inside an atomic() transaction block, showing a summary of changes that are made and then rolling the transaction back at the end. # 13th October 2022, 4:22 pm

The trouble with transaction.atomic (via) David Seddon provides a detailed explanation of Django’s nestable transaction.atomic() context manager and describes a gotcha that can occur if you lose track of whether your code is already running in a transaction block, since you may be working with savepoints instead—along with some smart workarounds. # 20th November 2020, 3:57 pm

Django now has fast tests. Changeset 9756 switched Django’s TestCase class to running tests inside a transaction and rolling back at the end (instead of doing a full dump and reload). “Ellington’s test suite, which was taking around 1.5-2 hours to run on Postgres, has been reduced to 10 minutes.” # 16th January 2009, 11:40 am

Django Unit Tests and Transactions. If you’re using a transactional database engine (MySQL with InnoDB, Postgres or SQLite) you can speed things up by running each of your unit tests inside a transaction and rolling back in tearDown(). # 7th July 2008, 2:14 pm