Simon Willison’s Weblog

Enforcing conventions in Django projects with introspection (via) Luke Plant shows how to use the Django system checks framework to introspect models on startup and warn if a DateTime or Date model field has been added that doesn’t conform to a specific naming convention.

Luke also proposes “*_at” as a convention for DateTimes, contrasting with “*_on” or “*_date” (I prefer the latter) for Dates. # 3rd April 2024, 2:58 pm

Talking about Django’s history and future on Django Chat (via) Django co-creator Jacob Kaplan-Moss sat down with the Django Chat podcast team to talk about Django’s history, his recent return to the Django Software Foundation board and what he hopes to achieve there.

Here’s his post about it, where he used Whisper and Claude to extract some of his own highlights from the conversation. # 21st March 2024, 12:42 am

DiskCache (via) Grant Jenks built DiskCache as an alternative caching backend for Django (also usable without Django), using a SQLite database on disk. The performance numbers are impressive—it even beats memcached in microbenchmarks, due to avoiding the need to access the network.

The source code (particularly in core.py) is a great case-study in SQLite performance optimization, after five years of iteration on making it all run as fast as possible. # 19th March 2024, 3:43 pm

Django Chat: Datasette, LLMs, and Django. I’m the guest on the latest episode of the Django Chat podcast. We talked about Datasette, LLMs, the New York Times OpenAI lawsuit, the Python Software Foundation and all sorts of other topics. # 24th January 2024, 8:41 pm

DSF calls for applicants for a Django Fellow. The Django Software Foundation employs contractors to manage code reviews and releases, responsibly handle security issues, coach new contributors, triage tickets and more.

This is the Django Fellows program, which is now ten years old and has proven enormously impactful.

Mariusz Felisiak is moving on after five years and the DSF are calling for new applicants, open to anywhere in the world. # 20th January 2024, 8:35 am

Database generated columns: GeoDjango & PostGIS. Paolo Melchiorre advocated for the inclusion of generated columns, one of the biggest features in Django 5.0. Here he provides a detailed tutorial showing how they can be used with PostGIS to create database tables that offer columns such as geohash that are automatically calculated from other columns in the table. # 11th December 2023, 7:14 pm

Database Migrations. Vadim Kravcenko provides a useful, in-depth description of the less obvious challenges of applying database migrations successfully. Vadim uses and likes Django’s migrations (as do I) but notes that running them at scale still involves a number of thorny challenges.

The biggest of these, which I’ve encountered myself multiple times, is that if you want truly zero downtime deploys you can’t guarantee that your schema migrations will be deployed at the exact same instant as changes you make to your application code.

This means all migrations need to be forward-compatible: you need to apply a schema change in a way that your existing code will continue to work error-free, then ship the related code change as a separate operation.

Vadim describes what this looks like in detail for a number of common operations: adding a field, removing a field and changing a field that has associated business logic implications. He also discusses the importance of knowing when to deploy a dual-write strategy. # 1st October 2023, 11:55 pm

Building Search DSLs with Django (via) Neat tutorial by Dan Lamanna: how to build a GitHub-style search feature—supporting modifiers like “is:open author:danlamanna”—using PyParsing and the Django ORM. # 19th June 2023, 8:30 am

Writing a chat application in Django 4.2 using async StreamingHttpResponse, Server-Sent Events and PostgreSQL LISTEN/NOTIFY (via) Excellent tutorial by Víðir Valberg Guðmundsson on implementing chat with server-sent events using the newly async-capable StreamingHttpResponse from Django 4.2x.

He uses PostgreSQL’a LISTEN/NOTIFY mechanism which can be used asynchronously in psycopg3—at the cost of a separate connection per user of the chat.

The article also covers how to use the Last-Event-ID header to implement reconnections in server-sent events, transmitting any events that may have been missed during the time that the connection was dropped. # 19th May 2023, 3:42 pm

Django 4.2 released. “This version has been designated as a long-term support (LTS) release, which means that security and data loss fixes will be applied for at least the next three years.” Some neat new async features, including improvements to async streaming responses. # 3rd April 2023, 2:14 pm

djngo.com: Portable Django (via) “A 20mb executable zip file with Python 3.6 and Django 2.2. Works on Windows, Linux, MacOSX with x86_64 and aarch64 (yes, Apple M1 and Raspberry Pi).” The latest wizardry from the ecosystem surrounding the Cosmopolitan project, which provides a should-be-impossible mechanism for running the same executable on a bunch of different platforms. This utility by Ariel Núñez bundles Python and Django and SQLite, such that a Django application can become a portable executable ready to run on multiple platforms. It’s currently limited to Python 3.6 and Django 2.2 since those are the versions that run under Cosmopolitan, but I expect we’ll see more recent versions of those dependencies in the future. # 24th February 2023, 12:52 am

How to simulate a broken database connection for testing in Django (via) Neil Kakkar explores the options using unittest.patch() and then settles on a neater pattern using “with connection.execute_wrapper(QueryTimeoutWrapper()):” to simulate the exact exception he needs to test against. # 15th January 2023, 8:31 pm

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

Black 22.1.0 (via) Black, the uncompromising code formatter for Python, has had its first stable non-beta release after almost four years of releases. I adopted Black a few years ago for all of my projects and I wouldn’t release Python code without it now—the productivity boost I get from not spending even a second thinking about code formatting and indentation is huge.

I know Django has been holding off on adopting it until a stable release was announced, so hopefully that will happen soon. # 30th January 2022, 1:23 am

How to Add a Favicon to Your Django Site (via) Adam Johnson did the research on the best way to handle favicons—Safari still doesn’t handle SVG icons so the best solution today is a PNG served from the /favicon.ico path. This article inspired me to finally add a proper favicon to Datasette. # 20th January 2022, 7:03 am

django-upgrade (via) Adam Johnson’s new CLI tool for upgrading Django projects by automatically applying changes to counter deprecations made in different versions of the framework. Uses the Python standard library tokenize module which gives it really quick performance in parsing and rewriting Python code. Exciting to see this kind of codemod approach becoming more common in Python world—JavaScript developers use this kind of thing a lot. # 26th September 2021, 5:42 am

Django SQL Dashboard 1.0 (via) As part of my ongoing attempt to be braver about 1.0 releases (crucial if you want to do semantic versioning properly) I’ve released version 1.0 of Django SQL Dashboard, my Datasette-inspired app for Django that adds an interface for running read-only, bookmarkable SQL queries against a PostgreSQL database. The new version adds a column cog menu providing shortcuts for changing the sort order, counting distinct values and performing a group-by/count against column values. # 1st July 2021, 5:44 pm

Django for Startup Founders: A better software architecture for SaaS startups and consumer apps (via) The opening section of this article has very little to do with Django: it’s an insightful description of the technical challenges faced by a startup that is still seeking product-market fit. Alex then extends that into his own architectural recommendations for startups building with Django to help waste as little time as possible on problems that aren’t core to the product they are building. # 24th June 2021, 8:43 pm

HackSoft Django styleguide: services and selectors. HackSoft’s Django styleguide uses the terms “services” and “selectors”. Services are functions that live in services.py and perform business logic operations such as creating new entities that might span multiple Django models. Selectors live in selectors.py and perform more complex database read operations, such as returning objects in a way that respects visibility permissions. # 24th May 2021, 7:17 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

Bedrock: The SQLitening (via) Back in March 2018 www.mozilla.org switched over to running on Django using SQLite! They’re using the same pattern I’ve been exploring with Datasette: their SQLite database is treated as a read-only cache by their frontend servers, and a new SQLite database is built by a separate process and fetched onto the frontend machines every five minutes by a scheduled task. They have a healthcheck page which shows the latest version of the database and when it was fetched, and even lets you download the 25MB SQLite database directly (I’ve been exploring it using Datasette). # 7th October 2020, 11:47 pm

Pysa: An open source static analysis tool to detect and prevent security issues in Python code (via) Interesting new static analysis tool for auditing Python for security vulnerabilities—things like SQL injection and os.execute() calls. Built by Facebook and tested extensively on Instagram, a multi-million line Django application. # 7th August 2020, 8:50 pm

James Bennett on why Django should not support JWT in core (via) The topic of adding JWT support to Django core comes up occasionally—here’s James Bennett’s detailed argument for not doing that. The short version is that the JWT specification isn’t just difficult to implement securely: it’s fundamentally flawed, which results in things like five implementations in three different languages all manifesting the same vulnerability. Third party modules exist that add JWT support to Django, but baking it into core would act as a form of endorsement and Django’s philosophy has always been to encourage people towards best practices. # 1st August 2020, 12:28 am

PostgreSQL full-text search in the Django Admin. Today I figured out how to use PostgreSQL full-text search in the Django admin for my blog, using the get_search_results method on a subclass of ModelAdmin. # 25th July 2020, 11:05 pm

How to find what you want in the Django documentation (via) Useful guide by Matthew Segal to navigating the Django documentation, and tips for reading documentation in general. The Django docs have a great reputation so it’s easy to forget how intimidating they can be for newcomers: Matthew emphasizes that docs are rarely meant to be read in full: the trick is learning how to quickly search them for the things you need to understand right now. # 3rd July 2020, 3:04 pm

Django Release Cycle (via) Really nice visual representation of Django’s release cycle, built by Jeff Triplett as a remix of the Python release cycle by Dustin Ingram. # 3rd April 2020, 4:56 pm

Django: Added support for asynchronous views and middleware (via) An enormously consequential feature just landed in Django, and is set to ship as part of Django 3.1 in August. Asynchronous views will allow Django applications to define views using “async def myview(request)”—taking full advantage of Python’s growing asyncio ecosystem and providing enormous performance improvements for Django sites that do things like hitting APIs over HTTP. Andrew has been puzzling over this for ages and it’s really exciting to see it land in a form that should be usable in a stable Django release in just a few months. # 19th March 2020, 3:43 am

New governance model for the Django project. This has been under discussion for a long time: I’m really excited to see it put into action. It’s difficult to summarize, but they key effect should be a much more vibrant, active set of people involved in making decisions about the framework. # 12th March 2020, 5:27 pm

The first ever commit to Sentry (via) This is fascinating: the first 70 lines of code that started the Sentry error tracking project. It’s a straight-forward Django process_exception() middleware method that collects the traceback and the exception class and saves them to a database. The trick of using the md5 hash of the traceback message to de-dupe errors has been there from the start, and remains one of my favourite things about the design of Sentry. # 6th November 2019, 11:08 pm

Using dependabot to bump Django on my blog from 2.2 to 2.2.1 (via) GitHub recently acquired dependabot and made it free, and I decided to try it out on my blog. It’s a really neat piece of automation: it scans your requirements.txt (plus a number of other packaging definitions across several different languages), checks for updates to your dependencies and opens pull requests against any that it finds. Combine it with a CI service such as Circle CI and your tests will run automatically against the pull request, letting you know if it’s safe to merge. dependabot constantly rebases other changes against the pull request to try and ensure it will merge as cleanly as possible. # 27th May 2019, 1:24 am