For completeness, simply running python gives

error: Target Python binary 'python' not found in project. Most likely running 'rye sync' will resolve this.

What was the reasoning behind this default behaviour?

I am also confused by the inconsistent behavior of rye shims.

A toolchain seems to be working fine with behavior.global-python=true, but rye suddenly refuses to use it once it finds an empty file named pyproject.toml:

which python3
# /Users/watal/.rye/shims/python3
python3 -c "import sys; print(sys.prefix)"
# /Users/watal/.rye/py/[email protected]
touch pyproject.toml
python3 -c "import sys; print(sys.prefix)"
# /Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9

I'm gonna add to this. The global behavior isn't very good. I have to do lots of work arounds to make it work.

It seems @bayashi-cl has it right with #1122: for python projects not managed by rye (so no tool.rye.managed = true), rye should probably use global python all the time, through global shims if activated.

I agree that the current behavior is not ideal but I think rather than patching around on the behavior it would be better to take a step back and rethink what the actual intended behavior should be for all cases.