Let's start with...
>>> import this
The Zen of Python, by Tim Peters
Beautiful is better than ugly.
...
Readability counts.
Special cases aren't special enough to break the rules.
I recommend to choose the approach that is not questionable. It usually means something that is self-explainable, simple and understandable. If you insist on expressing the not used, what about the name unused or dummy?
Should the _ be more invisible, hidden or even the candidate for to be overlooked, if possible?
For Perl users, the _ means something else than unused. Even though you may not care about the Perl users, what would the _ mean for you if you never met it before?
