Artifact: One Falsifiable Hypothesis Before the Debugger

ex-23 · exercises co-13 · expected-vs-actual, then exactly one hypothesis, checked once.

Bug: apply_discount(100.0, 0.2) returns 20.0. Expected: 80.0.

Expected: apply_discount(100.0, 0.2) == 80.0  (a 20% discount KEEPS 80% of the total)
Actual:   apply_discount(100.0, 0.2) == 20.0
 
Hypothesis: the function returns `total * pct` (the DISCOUNT amount) instead of
`total * (1 - pct)` (the amount KEPT after the discount) -- i.e. the two quantities are swapped.
 
One check: read discount.py's return line directly.
  return total * pct     <- confirms the hypothesis exactly; no other check needed.

Verify: exactly one hypothesis is stated, and exactly one check (reading the single return line) either confirms or refutes it -- no debugger session, no scattershot print statements across the whole call stack, satisfying co-13's discipline.

Key takeaway: a hypothesis narrow enough to be wrong in one specific way needs only one check to resolve -- the discipline is in narrowing the guess before looking, not in how the bug is eventually found.

Why It Matters: undisciplined debugging (add a print statement everywhere, rerun, stare at the output) finds bugs eventually but generates noise and rarely produces a reusable insight; one falsifiable hypothesis, confirmed or refuted by exactly one targeted check, is the fastest path AND leaves behind a clear, one-sentence explanation of what actually went wrong.