@mark.dickinson

> ...my expectation was that there would be few cases of breakage, and that for those few cases it shouldn't be difficult to fix the breakage.

This expectation is probably correct.

My issue is somewhat only partly on-topic here: If one wants to have all NaNs in one equivalency class (e.g. if used as a key-value for example in pandas) it is almost impossible to do so in a consistent way without taking a performance hit. It seems to be possible builtin-types (even if frozenset won't be pretty), but already something like


class A:
    def __init__(self, a):
        self.a=a
    def __hash__(self):
        return hash(self.a)
    def __eq__(self, other):
        return self.a == other.a

is not easy to handle.

A special comparator for containers would be an ultimative solution, but I see how this could be too much hassle for a corner case.