`uv sync` with upgraded system Python version fails to update `activate_this.py`

[andersk]

Summary

An uv virtual environment using the system Python version embeds a reference to the Python minor version number in activate_this.py.

$ docker run --rm -it ubuntu:22.04
# apt update
# apt install -y curl python3 python3.11
# curl -LsSf https://astral.sh/uv/install.sh | sh
# source $HOME/.local/bin/env
# uv init --no-pin-python app
# cd app
# uv add numpy
# grep site-packages .venv/bin/activate_this.py 
for lib in "../lib/python3.10/site-packages".split(os.pathsep):

If we then upgrade the system’s Python version (simulated here by changing the /usr/bin/python3 symlink, but an OS upgrade has the same effect), uv sync correctly installs wheels for the new Python version, but incorrectly leaves activate_this.py unchanged.

# ln -nsf python3.11 /usr/bin/python3
# uv sync
# uv run python -c 'import numpy; print(numpy)'
<module 'numpy' from '/app/.venv/lib/python3.11/site-packages/numpy/__init__.py'>
# grep site-packages .venv/bin/activate_this.py
for lib in "../lib/python3.10/site-packages".split(os.pathsep):

There are ways to force it to fix activate_this.py, e.g. uv sync -p python3.10; uv sync -p python3.11 (both commands are necessary). However, a simple uv sync ought to automatically update activate_this.py without further workarounds.

Platform

Ubuntu 22.04 amd64

Version

uv 0.6.9

Python version

Python 3.10.12

[zanieb]

I think uv sync should discard the virtual environment entirely in this case, I'm very surprised it does not?