Great quick intro, @willingc thanks! I think the special variable _ deserves more attention. It appears only alone in the last case clause, and then appears again in a discussion of *. Adding an example where _ appears inside a more elaborate pattern without * would clarify the essential meaning of _. Something like ('error', code, _).

I think it would help readers if that example could appear after an example with case _, but before any mention of *.

Also, as @gvanrossum suggested, it's important to have an example without case _ to make it clear that no match means no op.