This is closed, and maybe I am missing something. But from a general point of view, why does hashing of NaN not raise an error as it did for decimals, i.e. why was this not resolved exactly the other way around? I am mostly just wondering about this it is not a problem for me.

Hashing NaNs seems dangerous and surprising because it might work in dicts/sets, but normally doesn't. (The only thing for it to work right would be if NaN was a singleton, but that is impossible for subclasses, etc.).

The thing is:

In [16]: s = {float('nan'): 1, float('nan'): 2, float('nan'): 3}
In [17]: s
Out[17]: {nan: 1, nan: 2, nan: 3}

In [18]: s[float('nan')]
KeyError: nan

In [19]: n = float('nan')
In [20]: s = {n: 1, n: 2, n: 3}
In [21]: s
Out[21]: {nan: 3}

This is because `n is n`, and PyObject_RichCompareBool assumes that if `a is b` then `a == b` which is simply wrong for NaNs and also makes comparisons  of iterables including NaNs an impossible business. NaNs have their unavoidable weirdness, but at least for dictionaries/sets it would seem more clear to me if they raised an error.