They have every right to be confused. Who would guess that Python evaluates optional arguments once at compile-time? Heck, I've used Python for years and I wouldn't have been able to tell you off the top of my head how Python handles this code. That reminds me of a similar gotcha I've unfortunately run into on more than one occasion. # intialize a to be a list of 5 empty lists a = 5*[[]] # push a value onto the first list. a[0].append(1) What's a? --Mark (P.S. Stuff like this drives me nuts, which is why I find myself gravitating more and more to languages with a richer set of immutable data structures, e.g., Clojure/Scala/etc., where both of these gotchas would become a non-issue. I would love to see more immutable datatypes in Python other than numbers/strings/tuples, although realistically, I'd say that's unlikely to happen.)