I was thinking about use cases where one might want to preserve the original key. I was not however aware of PEP 455 and the reasons for it's rejection. My bad.

Yet It is still unclear to me what the best practice would be to reverse lookup unhashable objects. Say you have a list of objects from which you need the most common. If there is no good way of reversing the transform, your only option is iterating trough your list? Which defeats the purpose of using a Counter and you'd be better off counting explicitly. Maybe I'm imagining unrealistic/far-fetched use cases.