Pytest discipline

The one rule that zeroes your score

Never create, modify, move, or delete: test files, test data, conftest.py, pytest.ini,
tox.ini, setup.cfg [pytest] sections, pyproject.toml [tool.pytest...], or any pytest
plugin/config. Verify through running tests and through scratch scripts in /tmp — never by
adding tests to the repository. If a fix seems to require a test change, the fix is wrong.

Narrow verification (fast, cheap)

1. Reproduce the issue's snippet in /tmp/repro.py (write_file to /tmp/repro.py, then
   terminal cd /testbed && python /tmp/repro.py). Before your fix it should show the bug;
   after, it should pass. This is your private proof.
2. Run the existing tests that touch your change (you named them during orientation):
   terminal: cd /testbed && python -m pytest <file or node id> -x -q, timeout 180–300.
3. Read failures fully before changing code — most red sweeps point at your fix, not at the tests.

The regression sweep (mandatory before DONE)

Graders zero everything if any previously-passing test now fails. Budget ~1/4 of your time:

1. Run the test files most related to your change first (tests/test_<module>*).
2. Then run the widest suite you can afford, at least the few files around your module:
   cd /testbed && python -m pytest tests/ -q -x --timeout=<n> if the project supports timeouts,
   else plain -q. Keep timeout under 300 per call; if the full suite is too slow, run the
   files for the packages you touched and say so.
3. If a test fails: is it your fix? Narrow it (prefer the smallest diff that keeps your repro
   green AND the neighbor tests green). Is it failing without your fix too (pre-existing)?
   Verify by git stash && python -m pytest <that test> -q && git stash pop — wait, git here has
   no history: instead patch your change out mentally, or keep a copy of the original file in
   /tmp before editing (terminal: cp file /tmp/file.orig) and diff/restore from there.
4. Keep a pristine copy BEFORE the first edit of any file you touch:
   terminal: cp path/to/file.py /tmp/orig-name.py. Restoring is cp back. This is your undo.

End state checklist (all must hold before DONE)

• /tmp/repro.py shows the issue fixed.
• The neighboring existing tests pass.
• The sweep found no NEW failures (pre-existing failures: leave alone, mention in your final note).
• git status (the repo is a single-commit repo) shows ONLY source files changed — no test
  files, no config files, no stray files in the repo.
• No new files were created inside /testbed except source the fix strictly required.