Simon Willison’s Weblog

19th July 2024 - Link Blog

GitHub Actions: Faster Python runs with cached virtual environments (via) Adam Johnson shares his improved pattern for caching Python environments in GitHub Actions.

I've been using the pattern where you add cache: pip to the actions/setup-python block, but it has two disadvantages: if the tests fail the cache won't be saved at the end, and it still spends time installing the packages despite not needing to download them fresh since the wheels are in the cache.

Adam's pattern works differently: he caches the entire .venv/ folder between runs, avoiding the overhead of installing all of those packages. He also wraps the block that installs the packages between explicit actions/cache/restore and actions/cache/save steps to avoid the case where failed tests skip the cache persistence.