Why return NotImplemented instead of raising NotImplementedError

It’s because __lt__() and related comparison methods are quite commonly used indirectly in list sorts and such. Sometimes the algorithm will choose to try another way or pick a default winner. Raising an exception would break out of the sort unless caught, whereas NotImplemented doesn’t get raised and can be used in further tests.

http://jcalderone.livejournal.com/32837.html

To summarise that link:

“NotImplemented signals to the runtime that it should ask someone else to satisfy the operation. In the expression a == b, if a.__eq__(b) returns NotImplemented, then Python tries b.__eq__(a). If b knows enough to return True or False, then the expression can succeed. If it doesn’t, then the runtime will fall back to the built-in behavior (which is based on identity for == and !=).”