R. David Murray added the comment: Unless I'm misremembering, it is exactly __lt__ (or __gt__, if __lt__ returns NotImplemented) that sorting depends on. Since I'm sure there is code out there that depends on this fact, I wonder if it should be part of the language definition. Also, the comparison documentation (https://docs.python.org/3/reference/expressions.html#comparisons) speaks about "total ordering" as being the requirement, which has a specific mathematical meaning (which sets, for example, do not satisfy, even though they have a __lt__ method). Whether or not the distinction is worth explaining in the tutorial is a open question. ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue14050> _______________________________________