@nguyendat I'm afraid this test case doesn't really make sense, and neither does the discussion on that exercise: the purpose of a default value is to be used only if no argument whatsoever is supplied for that parameter ... if any string-convertible argument is provided the argument should be used, even if it's nonsensical, otherwise you're second guessing your user's intent, and the user is allowed to have nonsensical intent.

So the "correct" response for two_fer("") really is "One for , one for me." ... it might not make the most sense, but it's more consistent than assuming that an explicitly passed empty string is the same thing as no string being passed at all. Similarly two_fer(None) really should return "One for None, one for me." ... if the user explicitly passes an argument we should tend to assume that was an informed choice, even if it's a silly one.

I'm going to close this as wontfix for that reason, but thank you for the contribution.