[Python-Dev] decimal.py: == and != comparisons involving NaNs (original) (raw)

Stefan Krah stefan-usenet at bytereef.org
Mon Nov 9 14:21:24 CET 2009

Antoine Pitrou <solipsis at pitrou.net> wrote:

> I see the point, but Decimal("NaN") does not hash:

Ok but witness again: >>> L = [1, 2, Decimal("NaN"), 3] >>> 3 in L True >>> class H(object): ... def eq(self, other): raise ValueError ... >>> L = [1, 2, H(), 3] >>> 3 in L Traceback (most recent call last): File "", line 1, in File "", line 2, in eq ValueError

Yes, but the list is already broken in two ways:

L.sort() Traceback (most recent call last): File "", line 1, in File "/usr/lib/python2.7/decimal.py", line 877, in lt ans = self._compare_check_nans(other, context) File "/usr/lib/python2.7/decimal.py", line 782, in _compare_check_nans self) File "/usr/lib/python2.7/decimal.py", line 3755, in _raise_error raise error(explanation) decimal.InvalidOperation: comparison involving NaN

Decimal("NaN") in L False

(NB: interestingly, float("nan") does hash)

I wonder if it should:

d = {float('nan'): 10, 0: 20} 0 in d True float('nan') in d False d[float('nan')] Traceback (most recent call last): File "", line 1, in KeyError: nan

I guess my point is that NaNs in lists and dicts are broken in so many ways that it might be good to discourage this use. (And get the added benefit of safer mathematical behavior for == and !=.)

Stefan Krah

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