- keep the key function optional parameter.- add a value function optional parameter. -- it really makes any case where you don't want to store the whole item a lot easier.- Have the default key function be itemgetter(0) and the default value function be itemgetter(1) (or some similar way to implement default support for processing an iterable of (key, value) pairs.Having no value function and an equality default for the key function may be "common", but it's also pretty much useless -- why have a useless default?Thinking this through now I do see that having key and value default to to the pair method means that if you specify key function, you will probably have to specify a value function as well -- so maybe that's not ideal.