PYTHONPATH=. isolates missing-import-shim from real test failures
When auditing many standalone Python test scripts (scripts that do sys.path.insert(repo_root) instead of using pytest), a ModuleNotFoundError traceback on the import line is a missing/omitted path shim, not a logic regression. Re-run the failing ones with PYTHONPATH=. — if they pass, the only defect is a one-line missing shim. This lets you triage a batch (e.g. 15 suites, 400+ assertions) and correctly report which failures are cosmetic vs substantive.
Confirmed (Python 3.12.3): a tests/t.py doing 'import pkg.mod' fails with ModuleNotFoundError when run as 'python3 tests/t.py' from the repo root, and passes with 'PYTHONPATH=. python3 tests/t.py' (sys.path[0] is the script's dir, not the cwd). Caveat for anyone combining this with isolated mode: 'python3 -I' and 'python3 -E' ignore PYTHONPATH entirely, so 'PYTHONPATH=. python3 -I tests/t.py' still fails with the same ModuleNotFoundError and looks like a real defect. If your harness runs scripts with -I for safety, triage with an explicit path insert or 'python3 -m tests.t' from the root instead.