Syllabus Lesson 98 of 141 · Devil's advocate
Devil's advocate

The pre-mortem

Why it matters Asking "what could go wrong?" gets polite, vague answers. Asking "it went wrong: why?" gets specific ones, because you are explaining a story instead of guessing a risk.

Pre-mortem: before launch, imagine the project has already failed, then write down the reasons. Each reason is a risk you can check for now.

Marketing launches a voucher campaign on Monday. The voucher service is small:

v = make()
v.add("SAVE10", uses=500, expires=30)   # valid until day 30, inclusive
v.redeem("SAVE10", "ann", today=3)       # True: accepted
v.redeem("SAVE10", "ann", today=4)       # False: once per customer

It is a month later and the campaign was a disaster. Write the reasons. A sparring prompt that helps: "This voucher launch failed badly. Write five different post-incident summaries." Then turn every reason into a check. Six hidden versions of the service each have one fault injected; your checks run against all of them.

Your turn

Set premortem to a list of at least 4 reasons the launch failed (plain sentences). Then write check_launch(make): make() returns a fresh voucher service, and your function must raise if it breaks a rule. The rules: add(code, uses, expires) creates a code; redeem(code, user, today) returns True or False; codes are not case sensitive; each user can redeem a code once; a code is accepted at most uses times; it works up to and including day expires, not after; a refused attempt uses nothing up; an unknown code returns False (it never raises). Pass bar: pass the correct service and catch at least 4 of the 6 faulty ones.

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