docs: describe the test filesystem guard as it actually behaves - #348
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docs: describe the test filesystem guard as it actually behaves#348MrJarnould wants to merge 2 commits into
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CONTRIBUTING.md and AGENTS.md both say "new tests must mock any file I/O". Neither the guard nor the suite works that way, and the gap has produced the same review finding three times against tests that were using the guard exactly as intended. What the guard really does, established by testing it rather than reading it: - `open`, `os.open`, `os.mkdir`, `os.makedirs` and `os.chmod` raise unless the path contains `python-test-results`. That marker is a sanctioned escape hatch, not a loophole; tests/test_cmdline.py uses it for session directories in all seventy-odd of its API fixtures. - `Path.read_text()` is never intercepted. It goes through `io.open`, and only `builtins.open` is patched. This is why loading a JSON fixture works, and it works inside a test body too -- so the import-time ordering of the session-scoped fixtures is not the whole explanation, which is what I assumed in timlaing#333. AGENTS.md already documented the escape hatch and then contradicted itself in the next sentence. CONTRIBUTING.md did not mention it at all. tests/test_conftest_guard.py pins each of these, so the documentation and the guard cannot drift apart again without something failing. Closes timlaing#333 Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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📝 SummarySummary by CodeRabbit
WalkthroughThe change documents filesystem guard behaviour, including its sanctioned path and deliberate exceptions. New tests verify blocked operations, permitted temporary-directory access, and unIntercepted ChangesFilesystem guard behaviour
Estimated code review effort: 2 (Simple) | ~10 minutes Merge Risk: 🔵 Low · up to This change documents and tests filesystem guard behavior, but two documented guarded APIs lack contract coverage. Add coverage for os.open and os.chmod to prevent future guard/documentation drift. 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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In `@tests/test_conftest_guard.py`:
- Around line 29-35: Add test coverage in
test_open_is_blocked_outside_the_sanctioned_path and the adjacent guard tests
for both os.open and os.chmod, verifying blocked paths raise
FileSystemAccessError. If the sanctioned-path escape hatch is intended to be
uniform, also verify permitted paths for these APIs using the existing test
patterns.
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AGENTS.mdCONTRIBUTING.mdtests/test_conftest_guard.py
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The guard test exercised `open`, `os.mkdir` and `os.makedirs`, but the documents it exists to hold honest name five entry points. A regression in `os.open` or `os.chmod` would have left both documents inaccurate while these tests stayed green -- which is the exact drift this file was added to prevent. Both are now covered. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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Proposed change
CONTRIBUTING.mdandAGENTS.mdboth end their description of the test filesystem guardwith "New tests must mock any file I/O." Neither the guard nor the suite works that way, and
the gap is not academic: the same review finding has now been raised three times — on #336,
#339 and #346 — against tests that were using the guard exactly as intended. One review
thread is still held open pending this.
I established what the guard does by testing it rather than reading it:
openinside a testPath.read_text()inside a testos.mkdiroutsidepython-test-resultsos.mkdirinsidepython-test-resultsTwo things follow, and the docs mention neither correctly:
python-test-resultsis a sanctioned escape hatch, not a loophole.tests/test_cmdline.pyuses it for the session directory behind all seventy-odd of its API fixtures. A reviewer
reading only the absolute rule calls that a violation.
Path.read_text()is never intercepted. It goes throughio.open, and onlybuiltins.openis patched. This is why loading a JSON fixture works — and it works inside atest body as well as at import time. That corrects my own claim in #333, where I said the
cause was module-level code running before the autouse fixtures; the import-time ordering is
real for the session-scoped guards, but it is not the whole explanation and not the load-bearing
part. Seven test modules depend on this.
AGENTS.mdalready documented the escape hatch and then contradicted itself in the followingsentence.
CONTRIBUTING.mddid not mention it at all.The test is the point
tests/test_conftest_guard.pypins each documented behaviour:openandmkdirblockedoutside the marker, the marker permitted, and
pathlibreads not intercepted. Documentationthat nothing enforces is how this drifted in the first place, so the two now fail together or
not at all.
Type of change
Additional information
Cut from
main, touching only the two documents and one new test file. Independent of #345,#346 and #347.
On the alternative. #333 offered a second route: close the gap instead, by patching
Path.read_texttoo, so the absolute rule becomes literally true. That is defensible, but itbreaks fixture loading in seven test modules and the repository ships a documented
tests/fixtures/tree built for exactly this pattern — so it looked like changing the code tomatch a sentence rather than the other way round. Happy to do that version instead if you
prefer it; it is a much larger diff and I did not want to guess.
Testing. 937 tests pass on Python 3.10, 3.11, 3.12, 3.13 and 3.14, run locally. Four are
new.
Checklist
If user exposed functionality or configuration variables are added/changed:
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