Simon Willison’s Weblog

time-machine example test for a segfault in Python (via) Here’s a really neat testing trick by Adam Johnson. Someone reported a segfault bug in his time-machine library. How you you write a unit test that exercises a segfault without crashing the entire test suite?

Adam’s solution is a test that does this:

subprocess.run([sys.executable, “-c”, code_that_crashes_python], check=True)

sys.executable is the path to the current Python executable—ensuring the code will run in the same virtual environment as the test suite itself. The -c option can be used to have it run a (multi-line) string of Python code, and check=True causes the subprocess.run() function to raise an error if the subprocess fails to execute cleanly and returns an error code.

I’m absolutely going to be borrowing this pattern next time I need to add tests to cover a crashing bug in one of my projects. # 23rd March 2024, 7:44 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

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

Better Python Decorators with wrapt (via) Adam Johnson explains the intricacies of decorating a Python function without breaking the ability to correctly introspect it, and dicsusses how Scout use the wrapt library by Graham Dumpleton to implement their instrumentation library. # 2nd July 2020, 9:48 pm