Python preference option system disables python downloads

[Olindholm]

I would like uv to prioritize using system python when possible, but download a managed python version if necessary.

I figure using the --python-preference system option (or export UV_PYTHON_PREFERENCE=system) would do this. However it seems when using that, downloads are disabled.

$ uv python list
cpython-3.13.0+freethreaded-linux-x86_64-gnu    <download available>
cpython-3.12.7-linux-x86_64-gnu                 <download available>
cpython-3.11.10-linux-x86_64-gnu                <download available>
cpython-3.10.15-linux-x86_64-gnu                <download available>
cpython-3.10.12-linux-x86_64-gnu                /usr/bin/python3.10
cpython-3.10.12-linux-x86_64-gnu                /usr/bin/python3 -> python3.10
cpython-3.10.12-linux-x86_64-gnu                /usr/bin/python -> python3
cpython-3.10.12-linux-x86_64-gnu                /bin/python3.10
cpython-3.10.12-linux-x86_64-gnu                /bin/python3 -> python3.10
cpython-3.10.12-linux-x86_64-gnu                /bin/python -> python3
cpython-3.9.20-linux-x86_64-gnu                 <download available>
cpython-3.8.20-linux-x86_64-gnu                 <download available>
cpython-3.7.9-linux-x86_64-gnu                  <download available>
pypy-3.10.14-linux-x86_64-gnu                   <download available>
pypy-3.9.19-linux-x86_64-gnu                    <download available>
pypy-3.8.16-linux-x86_64-gnu                    <download available>
pypy-3.7.13-linux-x86_64-gnu                    <download available>
$ uv venv --python 3.8 --python-preference system --verbose
DEBUG uv 0.4.29
DEBUG Searching for Python 3.8 in managed installations or system path
DEBUG Found `cpython-3.10.12-linux-x86_64-gnu` at `/usr/bin/python` (search path)
DEBUG Skipping interpreter at `/usr/bin/python` from search path: does not satisfy request `3.8`
DEBUG Found `cpython-3.10.12-linux-x86_64-gnu` at `/usr/bin/python3` (search path)
DEBUG Skipping interpreter at `/usr/bin/python3` from search path: does not satisfy request `3.8`
DEBUG Found `cpython-3.10.12-linux-x86_64-gnu` at `/bin/python` (search path)
DEBUG Skipping interpreter at `/bin/python` from search path: does not satisfy request `3.8`
DEBUG Found `cpython-3.10.12-linux-x86_64-gnu` at `/bin/python3` (search path)
DEBUG Skipping interpreter at `/bin/python3` from search path: does not satisfy request `3.8`
DEBUG Searching for managed installations at `/home/olindho1/.local/share/uv/python`
DEBUG Skipping incompatible managed installation `cpython-3.12.6-linux-x86_64-gnu`
  × No interpreter found for Python 3.8 in managed installations or system path

I don't see this behavior being described anywhere. Is this intentional or a bug?

[zanieb]

Thanks for the report!