Simon Willison: if in doubt, delete the uv.lock file to fix Python 3.15 dependency errors
Simon Willison · rss · 2026-11-02
Simon Willison shares a TIL: since switching to Python 3.15 he occasionally hits confusing dependency build errors, and the rule is "if in doubt, delete the uv.lock file."
Most of his projects are libraries that shouldn't ship a lock file, but commands like uv run pytest leave uv.lock files behind (gitignored, never committed). These stale lock files can pin a dependency version lacking a Python 3.15 wheel — e.g. watchfiles==1.1.1 failing because PyO3 only supports up to Python 3.14. Deleting the file and re-running the command usually fixes it.
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