pip 26.1 Introduces Lockfiles and Dependency Cooldowns

Python's package installer pip has released version 26.1, featuring lockfiles and dependency cooldowns. The update drops support for Python 3.9, which reached end-of-life in October. The new lockfile feature generates a `pylock.toml` file, as demonstrated by installing Datasette and LLM, producing a 519-line lockfile. Dependency cooldowns are implemented via the `--uploaded-prior-to` option using ISO duration format (e.g., `P4D` for 4 days). LLM version 0.31, released three days prior, was used to test the cooldown feature. The announcement was made by Richard Si and shared via a link post by Simon Willison on April 28, 2026.