On python-ideas, thread "Optional key to `bisect`'s functions?" Guido wrote "Bingo. That clinches it. We need to add key=." 'That' being the fact that values that have keys may not be comparable themselves (in py3), so that comparing (key,value) pairs may raise TypeError. This follows a reply by him yesterday saying that he has wanted this feature occasionally. I am re-opening this rather than a new issue because there is already a patch with tests.