Your unit test should exercise the whole API promised by an implementation anyway, so often an explicit interface check is redudant (of course, it can't hurt). verifyClass() per se isn't bad, it's in fact a useful indicator, but having that it as a *sole* measure whether a class fulfills an interface or not is not sufficient (plus, in many Zope cases, verifyObject is better because attributes may only be initialized in __init__).
The point why I think it's YAGNI is that we know the Zope 2 implementations do implement the interfaces. After all, I derived the interfaces from the implementations by gutting out the code. And it's unlikely they'll change (although I might be wrong on this one, in which case you win :)).
When writing "test-first", I often start with only the 'verifyClass' test, and an empty interface. Then as I flesh out the interface, the test fails, reminding me to add the method / attribute. Yes, you still need tests for the semantics, but the conformance test is still valuable, because it "tests the tests" (an extra safety belt).
Fair enough. Note the "extra": it shouldn't be your only one.
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