but the execution context for setuptools.pex is the source_root

What do you mean by this part? Normally, you’ll set

PEX_EXTRA_SYS_PATH

to handle source roots, which ends up appending to

PYTHONPATH

. https://github.com/pantsbuild/pants/blob/ea542ac164fb73459f7a7a90aff9c6646d2213e6/src/python/pants/backend/python/goals/pytest_runner.py#L227-L230 You can then run

./setuptools.pex ... src/python/path/to/f.py

, and it will appropriately handle the

src/python

source root.